Fix various things when n-cell is enabled

1) Use the newer cell_type configuration option instead of overriding
the compute_api_class.

2) A nova-cells service is started for both the API cell (region) and
the compute cell (child).  The screen names were both 'n-cell' which
caused both services to log to the same log file.  The screen names have
been changed to n-cell-region and n-cell-child.

3) Security groups are not supported with cells.  Multiple exercises are
patched to use the 'default' security group instead of creating a new
one.  Additionally, if one uses the 'default' security group, do not try
to delete it, as one can never delete the default.

4) Disable the floating_ips and aggregates exercises when n-cell is
enabled, as they are not supported by cells.

5) Related to #4, disable the floating_ips tests within euca.sh

6) Update the README.md.  No services need to be disabled, and one only
needs to enable the n-cell service.

Change-Id: I9782d1e3cda3c9dd3daefa15c043f5b06473cb87
diff --git a/functions b/functions
index eb83dfb..08c5253 100644
--- a/functions
+++ b/functions
@@ -745,12 +745,17 @@
 #   For backward compatibility if we have **swift** in ENABLED_SERVICES all the
 #   **s-** services will be enabled. This will be deprecated in the future.
 #
+# Cells within nova is enabled if **n-cell** is in ``ENABLED_SERVICES``.
+# We also need to make sure to treat **n-cell-region** and **n-cell-child**
+# as enabled in this case.
+#
 # Uses global ``ENABLED_SERVICES``
 # is_service_enabled service [service ...]
 function is_service_enabled() {
     services=$@
     for service in ${services}; do
         [[ ,${ENABLED_SERVICES}, =~ ,${service}, ]] && return 0
+        [[ ${service} == n-cell-* && ${ENABLED_SERVICES} =~ "n-cell" ]] && return 0
         [[ ${service} == "nova" && ${ENABLED_SERVICES} =~ "n-" ]] && return 0
         [[ ${service} == "cinder" && ${ENABLED_SERVICES} =~ "c-" ]] && return 0
         [[ ${service} == "ceilometer" && ${ENABLED_SERVICES} =~ "ceilometer-" ]] && return 0