Exit Firewall Agent if config is invalid

When fwaas config file is not provided to the agent,
but the service is enabled in neutron.conf file the
agent should exit with an error message and should
not proceed further. This patch adds the necessary fix.

Change-Id: Iaced777e3a34e9405050252b17a203689e1c1fc0
Closes-Bug: #1310857
OVS flows apply concurrently using a deferred OVSBridge

This change is an improvement of the commit
501213686886baccd3280e10b8856a25d3517519 and provides a cleaner
implementation. Previously flows were applied on
OVSBridge.defer_apply_off which could be called by an other
greenthread: it was impossible to ensure that all flows are applied
in a unique OVSBridge.defer_apply_off call. This change ensures that
all flows defined using a DeferredOVSBridge are applied on
DeferredOVSBridge.apply_flows or DeferredOVSBridge.__exit__ if not
exception is raised.

Author:         Cedric Brandily <zzelle@gmail.com>
Co-Authored-By: Edouard Thuleau <edouard.thuleau@cloudwatt.com>

Related-bug: #1263866
Change-Id: I1f260629ef95b98ee80e2ff946c3606da8fe7608
Merge "Exit Firewall Agent if config is invalid"
Merge "OVS flows apply concurrently using a deferred OVSBridge"