blueprint agent-db-ha

blueprint agent-db-ha
bug 985470
bug 985646

The fixes enable the OVS and linuxbridge agenets to "keep alive" when the host running the server/plugin is down.

Fixes after comments. Better logging

Fixes after comments - added reconnect interval + cleanup

Fixes after comments - simplify code + ovs intervals moved to configuration file

Fixes after comments - move int conversion to configuration

Fixes after comments - if one of the polling interval or reconnect interval are not
defined in the relevant ini files then a default value is used.

Fixes after comments and merges with HACKING.rst fixes

Fixes after port binding comments

Fixes after comments from gongysh

Fixes after comments - align comments in agent ini files

Fixes - revert some code

Change-Id: I9194f142478b130e8ef198b019539357a9916d7f