Stale VXLAN & GRE tunnel endpoint deletion from DB

Stale VXLAN & GRE tunnel endpoint deletion from DB

Description:
Stale GRE and VXLAN tunnel endpoints persists in neutron db this should be
deleted from the database. Also, if local_ip of L2 agent changes the
stale tunnel ports and flows persists on br-tun on other Compute Nodes and
Network Nodes for that remote ip this should also be removed.

Implementation

Plugin changes:
Added host column in 'ml2_vxlan_endpoints' and 'ml2_gre_endpoints' table.
Added delete_endpoint method for deleting the stale endpoints from db.
Modified tunnel_sync() method to accommodate these changes.
Modified testcases in test_type_vxlan.py
Modified testcases in test_type_gre.py

Agent changes:
Added tunnel_delete rpc for removing stale ports and flows from br-tun.
tunnel_sync rpc signature upgrade to obtain 'host'.
Added testcases for TunnelRpcCallbackMixin().

This patch-set only deals with plugin side changes.

Partial-Bug: #1179223

Change-Id: I75c6581fcc9f47a68bde29cbefcaa1a2a082344e