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