Big Switch: Separate L3 functions into L3 service

Separate the L3 functions from the core Big Switch
plugin into an L3 service plugin. This is will allow
the L3 functions to be used in ML2 deployments that
use the Big Switch ML2 driver.

DocImpact

Implements: blueprint bsn-l3-service-plugin
Change-Id: I889db0047fb9a85f02d1fb95b9c099e9243a5bb5
Skip functional l3 agent test

Since this test merged there have been periodic failures that
seem to be related to SystemExit being raised.  Skipping for now
to give us time to fix.

Change-Id: Ia857835102a9c4c74820cf5eb3c529a3c2fb421e
Related-bug: 1364171
Implements ProcessMonitor to watch over external processes

This class takes care of all the spawned external processes,
taking the administrator configured action in the case of any
of the external processes die unexpectedly.

Implements: blueprint agent-child-processes-status

Change-Id: I6bc7a415dde5723dac07589859796c2ffeef5b54
Fix interface IP address for DVR with gateway

when an interface is added to a router with gateway, the method
internal_network_added(..) incorrectly configures the IP address
of the new snat interface same as the IP address of the new
subnet interface.

Change-Id: I00d4ff1105a443b89b28359249b8e83fb4c1d157
Closes-bug: #1355087
Merge "Big Switch: Separate L3 functions into L3 service"
Merge "Implements ProcessMonitor to watch over external processes"
Merge "Skip functional l3 agent test"
Merge "Fix interface IP address for DVR with gateway"