Add support for Nova live migrations

This patch adds support for live migration in a new live_migrate_server
benchmark scenario. Live migrate requires an admin context, as
does determining the host on which an instance is running. A special
is_ready function was added to check whether the instance host
record changed. This is because live migrations can fail and simply
return the instance to an active state, so the only way to know for
sure we have completed successfully is to check the host before and
after the operation is initiated.

Change-Id: I642f0ecc2b5c84202ebacd01e2b0c85ab609e597
Make endpoint type configurable

Rally is supposed to honor endpoint_type being defined as part of a
deployment configuration, which is currently broken. This patch
works around it by replacing all the hard coded passing of "public"
to make it a config option.

Change-Id: I759fc1bdff4e0e71ea4cb5576612d713fe84eb35
Disassociate Nova networks during cleanup

Closes-bug: #1350517

Add a dirty hack to deal with cleaning up associated Nova networks
after a Rally run. This was originally being done via the cleanup
classes but after discussing with Boris, it was determined this was
the least-bad way to approach the problem.

Change-Id: Ic52bb0d156dc2adebc9bca2daec660f81d16b31c
Fix live migration decorators from bad merge

Commit 70a90e01c969a813984089d9fc01bd0079d8295e changed the
decorators around most methods and I missed it during my merge.

Change-Id: I307b4c9c1700cb58e785241e478c5247a3e3b681