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
Update OpenVZ driver image

11.10 simply does not exist, so this simply does not work. However,
devstack-gate tries to pre-cache all images referenced, so even though
we're not doing anything with this, it's breaking devstack-gate.

Change-Id: I64b03ed387f5205a885427498e8cf8c2a5b32b91
xenapi: Uninstall instances

The devstack script sometimes failed to clean up the left over
instances. This patch will utilize the xe vm-uninstall force=true
command to get rid of the instances.

Change-Id: I3450e2d5c9a4b1fe8ede39c4cc5c337cd541ea35
Merge "Fix various things when n-cell is enabled"
Merge "Update OpenVZ driver image"
Merge "xenapi: Uninstall instances"