DVR: fix router rescheduling on agent side

DVR: fix router rescheduling on agent side

The patch makes L3 agent aware of possible SNAT role
rescheduling to/from it.
The gist is to compare gw_port host change.
If it was changed and agent is not on target host then
it needs to clear snat namespace if one exists. If agent
is on target host it needs to create snat namespace from
scratch if it doesn't exist.

Host field was excluded from gw_port comparison on
agent side as part of HA Router feature implementation.
This code was moved to corresponding module.

Closes-Bug: #1472205
Change-Id: I840bded9eb547df014c6fb2b4cbfe4a876b9b878