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
c62c2b9b66