Fix missing migration default value

Fix 128e042a2b68_ext_gw_mode and 40b0aff0302e_mlnx_initial
migrations by properly using server_default parameter in column
definitions.

Change-Id: I199e206dccc36fcfc2457a17167c76611d69784a
Closes-bug: #1336556
NSX: neutron router-interface-add should clear security-groups

NSX does not support security groups on router ports so in the case
where someone uses a port that has a security group on it as the router
port we need to clear the security group off the port.

Change-Id: Ia0fb331516887dcd7e9a435094ce1eb082d72575
closes-bug: 1329043
Extract CommonDBMixin to a separate file

db_base_plugin_v2 imports too much modules that are not necessary
usually, so extract CommonDBMixin in different file.
Plus using db_base_plugin_v2 for some types of modules can lead to
cycles in imports, this refactoring should resolve the issue.

Closes-Bug: #1340145
Change-Id: Idb027d7c5cee2d5bc7598f805c56c55fd4aca048
Merge "NSX: neutron router-interface-add should clear security-groups"
Merge "Fix missing migration default value"
BGP Dynamic Routing

First approach.