BigSwitch: Auto re-sync on backend inconsistencies

If the controller supports it, pass a hash to the
controller indicating the expected state that a
REST transaction is updating. If the state is
inconsistent, the controller will return an error
indicating a conflict and the plugin/driver will
trigger a full synchronization.

For controllers that don't support the consistency
hash, trigger a full background synchronization
if the plugin tries to create a port and receives
a 404 error due to the parent network not existing.

Implements: blueprint bsn-auto-resync
Change-Id: I07c92b011453f6bf81b8ee12661170817287cdd7
Merge "BigSwitch: Auto re-sync on backend inconsistencies"