Reduce security group db calls to neutron server

Within ovs agent daemon loop, prepare_devices_filter will impose heavy workloads
to neutron server in order to retrieve the security groups message to apply
firewall rules. If agent is configured to use Noopfirewall driver or security
groups are disabled, there is no need for loading the rules from server and
refreshing the firewalls. This will reduce the number of db calls and improve
performance for neutron server in this case.

Change-Id: Id244aab3cac37fc6ed3dc05cbee91cdf9e34d9cc
Closes-Bug: #1365806
VPNaaS Cisco unit test clean-up

Removed unused args to mock side-effect function and duplicate
constant.

Change-Id: I5409ce86ccaab86213d65f757f19c1bdf9a66929
Closes-Bug: #1381221
l3_agent: avoid name conflict with context

module name, context, conflicts with argument name in many place in
l3_agent. In order to avoid such conflict, import context as n_context
following Neutron practice.

Change-Id: Ic3754818f84064d2c8da04914826fc912437b2f0
Closes-Bug: #1381900
Schema enhancement to support MultiSegment Network

Description:
Currently, there is nothing in the schema that ensures segments
for a network are returned in the same order they were specified
when the network was created, or even in a deterministic order.

Solution:
We need to add another field named 'segment_index' in
'ml2_network_segment' table containing a numeric position index.
With segment_index field we can retrieve the segments in the
order in which user created.

This patch set also fixes ML2 invalid unit test case in
test_create_network_multiprovider().

Closes-Bug: #1224978
Closes-Bug: #1377346

Change-Id: I560c34c6fe1c5425469ccdf9b8b4905c123d496d
Merge "Schema enhancement to support MultiSegment Network"
Merge "l3_agent: avoid name conflict with context"
Merge "VPNaaS Cisco unit test clean-up"
Merge "Reduce security group db calls to neutron server"