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/configuration.rst b/doc/source/configuration.rst
index 22809eb..bc3f558 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -63,7 +63,7 @@
 ::
 
     [[local|localrc]]
-    FIXED_RANGE=10.254.1.0/24
+    IPV4_ADDRS_SAFE_TO_USE=10.254.1.0/24
     ADMIN_PASSWORD=speciale
     LOGFILE=$DEST/logs/stack.sh.log
 
@@ -161,8 +161,8 @@
 
 -  no logging
 -  pre-set the passwords to prevent interactive prompts
--  move network ranges away from the local network (``FIXED_RANGE`` and
-   ``FLOATING_RANGE``, commented out below)
+-  move network ranges away from the local network (``IPV4_ADDRS_SAFE_TO_USE``
+   and ``FLOATING_RANGE``, commented out below)
 -  set the host IP if detection is unreliable (``HOST_IP``, commented
    out below)
 
@@ -173,7 +173,7 @@
     DATABASE_PASSWORD=$ADMIN_PASSWORD
     RABBIT_PASSWORD=$ADMIN_PASSWORD
     SERVICE_PASSWORD=$ADMIN_PASSWORD
-    #FIXED_RANGE=172.31.1.0/24
+    #IPV4_ADDRS_SAFE_TO_USE=172.31.1.0/24
     #FLOATING_RANGE=192.168.20.0/25
     #HOST_IP=10.3.4.5
 
@@ -537,12 +537,12 @@
 
         IPV6_RA_MODE=slaac
         IPV6_ADDRESS_MODE=slaac
-        FIXED_RANGE_V6=fd$IPV6_GLOBAL_ID::/64
+        IPV6_ADDRS_SAFE_TO_USE=fd$IPV6_GLOBAL_ID::/56
         IPV6_PRIVATE_NETWORK_GATEWAY=fd$IPV6_GLOBAL_ID::1
 
-*Note*: ``FIXED_RANGE_V6`` and ``IPV6_PRIVATE_NETWORK_GATEWAY`` can be
-configured with any valid IPv6 prefix. The default values make use of
-an auto-generated ``IPV6_GLOBAL_ID`` to comply with RFC4193.
+*Note*: ``IPV6_ADDRS_SAFE_TO_USE`` and ``IPV6_PRIVATE_NETWORK_GATEWAY``
+can be configured with any valid IPv6 prefix. The default values make
+use of an auto-generated ``IPV6_GLOBAL_ID`` to comply with RFC4193.
 
 Service Version
 ~~~~~~~~~~~~~~~