Start openvswitch-switch early in upstart

When there is a bridge configuration in the interfaces definition the
openvswitch-switch service needs to be started early.

This ensures that the metadata server is available for cloud-init when
it runs.

Change-Id: Ieac3703dc910ccb420e1a5a1cdc40105bd4f067a
Closes-bug: #1314573
Merge "Start openvswitch-switch early in upstart"