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/exercises/aggregates.sh b/exercises/aggregates.sh
index 3c83725..e2baecd 100755
--- a/exercises/aggregates.sh
+++ b/exercises/aggregates.sh
@@ -42,6 +42,8 @@
 # Test as the admin user
 . $TOP_DIR/openrc admin admin
 
+# Cells does not support aggregates.
+is_service_enabled n-cell && exit 55
 
 # Create an aggregate
 # ===================