Derive IP ranges from new ADDRS_SAFE_TO_USE vars

The switch to using subnetpools caused quite a bit of confusion
because it didn't respect the value of FIXED_RANGE. This caused
conflicts in the gate with it's default IPv4 value of 10.0.0.0/8.

This patch does a few things to address the issue:
* It introduces the IPV4_ADDRS_SAFE_TO_USE and IPV6_ADDRS_SAFE_TO_USE
  values and adjusts all of the FIXED_RANGE and SUBNETPOOL_PREFIX values
  to dervive from them by default.
  * This addresses the concern that was raised about implying that
    SUBNETPOOL_PREFIX and FIXED_RANGE are equivalent when setting
    SUBNETPOOL_PREFIX=FIXED_RANGE by default. Now we have a new value
    for the operator specify a chunk of addresses that are safe to
    use for private networks without implementation implications.
  * Backwards compatibility is maintained by alloing users to override
    override all of these values.
* The default for IPV4_ADDRS_SAFE_TO_USE uses /22 instead of /24
  * Because we want to be able to use subnetpools for auto allocated
    topologies and we want to be able to have a large chunk of
    instances on each network, we needed a little more breathing room
    in the default v4 network size.
* SUBNET_POOL_SIZE_V4 default is changed from 24 to 26
  * In conjuction with this change and the one above, the default
    subnetpool will support up to 16 64-address allocations.
  * This should be enough to cover any regular gate scenarios.
  * If someone wants a bigger/smaller subnet, they can ask for that
    in the API request, change this value themselves, or use a different
    network entirely.
* FIXED_RANGE_V6 defaults to a max prefix of /64 from IPV6_ADDRS_SAFE_TO_USE
  * This avoids the private subnet in the non-subnetpool case from being
    larger than /64 to avoid issues identified in rfc 7421.
  * Users can still explicitly set this value to whatever they want.
    This 'max' behavior is only for the default.
  * This allows IPV6_ADDRS_SAFE_TO_USE to default to a /56, which leaves
    tons of room for v6 subnetpools.

Closes-Bug: #1629133
Change-Id: I7b32804d47bec743c0b13e434e6a7958728896ea
diff --git a/doc/source/guides/neutron.rst b/doc/source/guides/neutron.rst
index bc6816c..092809a 100644
--- a/doc/source/guides/neutron.rst
+++ b/doc/source/guides/neutron.rst
@@ -79,7 +79,7 @@
         ## Neutron options
         Q_USE_SECGROUP=True
         FLOATING_RANGE="172.18.161.0/24"
-        FIXED_RANGE="10.0.0.0/24"
+        IPV4_ADDRS_SAFE_TO_USE="10.0.0.0/22"
         Q_FLOATING_ALLOCATION_POOL=start=172.18.161.250,end=172.18.161.254
         PUBLIC_NETWORK_GATEWAY="172.18.161.1"
         PUBLIC_INTERFACE=eth0
@@ -387,17 +387,17 @@
 
         ## Neutron Networking options used to create Neutron Subnets
 
-        FIXED_RANGE="203.0.113.0/24"
+        IPV4_ADDRS_SAFE_TO_USE="203.0.113.0/24"
         NETWORK_GATEWAY=203.0.113.1
         PROVIDER_SUBNET_NAME="provider_net"
         PROVIDER_NETWORK_TYPE="vlan"
         SEGMENTATION_ID=2010
         USE_SUBNETPOOL=False
 
-In this configuration we are defining FIXED_RANGE to be a
+In this configuration we are defining IPV4_ADDRS_SAFE_TO_USE to be a
 publicly routed IPv4 subnet. In this specific instance we are using
 the special TEST-NET-3 subnet defined in `RFC 5737 <http://tools.ietf.org/html/rfc5737>`_,
-which is used for documentation.  In your DevStack setup, FIXED_RANGE
+which is used for documentation.  In your DevStack setup, IPV4_ADDRS_SAFE_TO_USE
 would be a public IP address range that you or your organization has
 allocated to you, so that you could access your instances from the
 public internet.
@@ -524,7 +524,7 @@
     ## Neutron options
     Q_USE_SECGROUP=True
     FLOATING_RANGE="172.18.161.0/24"
-    FIXED_RANGE="10.0.0.0/24"
+    IPV4_ADDRS_SAFE_TO_USE="10.0.0.0/24"
     Q_FLOATING_ALLOCATION_POOL=start=172.18.161.250,end=172.18.161.254
     PUBLIC_NETWORK_GATEWAY="172.18.161.1"
     PUBLIC_INTERFACE=eth0
@@ -573,7 +573,7 @@
     Q_AGENT=macvtap
     PHYSICAL_NETWORK=default
 
-    FIXED_RANGE="203.0.113.0/24"
+    IPV4_ADDRS_SAFE_TO_USE="203.0.113.0/24"
     NETWORK_GATEWAY=203.0.113.1
     PROVIDER_SUBNET_NAME="provider_net"
     PROVIDER_NETWORK_TYPE="vlan"