Create RPC connection before modifying OVS bridges

On startup, the agent removes and readds flows to the OVS bridges.  If
an RPC setup error exits the process prematurely, this can leave the
bridges in an unsafe state.  It is better to set the RPC communication
up before making changes to the host system.

Closes-Bug: 1217980
Change-Id: Ib9bbb864b9129bb7b1376a150a37a0c07908d74b
Signed-off-by: Stephen Gran <stephen.gran@guardian.co.uk>
Merge "Create RPC connection before modifying OVS bridges"