NSX: allow net-migration only in combined mode

To allow migration of networks from the agent based model
to the agentless one, both modes need to be available; prior
to this change the extension was available in 'agentless' mode
too; this may lead to leftovers during the migration step.
This patch makes the extension available in 'combined' mode
only, which was the intended behavior.

Closes-bug: #1313985

Change-Id: I491ee3eaf25fc30c0ab4092ee198802a6b06d91d
Common decorator for caching methods

Decorator caches methods' results, the key is composed from function
name and its parameters. Implementation of cache itself should be in
object implementing decorated method. Cache backend should be taken
from oslo-incubator.

Related-Bug: #1276440
Change-Id: Ibe3f1b2eb10b9601ad76ec24b7f0e3551b4013e2
Merge "Common decorator for caching methods"
Merge "NSX: allow net-migration only in combined mode"