Fix error adding security groups to instances with nexus

Adding security group to running instance should not
invoke the nexus plugin. Current implementation does not
validate for the 'new_host_id' and assumes it as host migration
and fails. As the nexus monolithic plugin has been removed
after icehouse, this would be merged to stable/icehouse.

Closes-Bug: #1365226

Change-Id: I9ab6b8438e6ee0698a8e2ae7ff7736f7c71b2370
Merge "Fix error adding security groups to instances with nexus" into stable/icehouse
Fix fallback stdout logging.

Fallback stdout logging should not be triggered if another log is setup.
Search for .pyc files to support .py file stripping.
avoid Duplicate key errors in ml2_gre_allocations