NSX: fix wording for configuration option

This patch clarifies that in order to use NSX distributed
routers, the replication_mode parameter should be set to
'service'. This is already its default value, so no further
change is needed beyond clarifying wording.

Change-Id: I446c428ecf8c0bf7f6947e69562d60a7fb4383ed
Closes-Bug: #1355502
Make plugin and l3plugin available as mixin's properties

This makes the code simpler and slightly more efficient.
This is a minor refactoring related to a race condition
observed when the agent is processing a router while its
interfaces are being removed.

Related-bug: #1355409

Change-Id: Ic0ce25aa6a977e28b1ab76081c74da4c46c2ca6b
Raise exception for network delete with subnets presents

N1kv plugin should raise an exception during network delete
if there is a subnet that is tied to that network.

Changed the order of the Assert arguments since now it's encoraged to have <expected_value, actual_value> now.

Change-Id: I92f177af0e2a2fbc460e0d668989ce590b7d1cb2
Closes-Bug: 1336107
Merge "Make plugin and l3plugin available as mixin's properties"
Merge "Raise exception for network delete with subnets presents"
Merge "NSX: fix wording for configuration option"