Fix random unit-test failure for NVP advanced plugin

Two problems were observed:
1. Tasks are not scheduled as desired in greenthread. The fix is to
   increase the max status probing time from 1 second to 10 seconds.
2. An error message is show in unit-test log output. Turns out in
   addition to a bug that may cause task manager stop prematurely,
   another unit-test case test_vcns_driver can cause same error
   message being displayed in the log.

Change-Id: I697dd8fc509308108ff1f40400f36ac6271bf4bb
Close-Bug: 1245698
Do not add DHCP info to subnet if DHCP is disabled

Currently the midonet plugin will add default DHCP information to
each subnet even if the subnet is configured to have DHCP disabled.
This change addresses this by checking the DHCP settings while
creating a subnet, and only adding DHCP information if DHCP is
enabled.

Closes-bug #1230073

Change-Id: I2c422866c60f505df30bfa18bfe8d03599665b7a
Rebind allowed address pairs only if they changed

This patch ensures allowed address pairs bindings are refreshed
only when they actually change.
This will also avoid sending a notification to the agent if no
change actually occured.

Closes-Bug: #1255145
Partial blueprint neutron-tempest-parallel

Change-Id: Iac2502586a0d215a29194590c16c2e1a064f943b
Add test_handle_router_snat_rules_add_rules

Add a unit test to verify that snat rules are added and added in order.
So that it could be refactored without fear next time.

Closes-Bug: #1222660
Change-Id: I07e820cc28c9a6c139a8eed0917aef2cfe62638a
Avoid loading policy when processing rpc requests

When Neutron server is restarted in the environment where multiple agents
are sending rpc requests to Neutron, it causes loading of policy.json
before API extensions are loaded. That causes different policy check
failures later on.
This patch avoids loading policy when creating a Context in rpc layer.

Change-Id: I66212baa937ec1457e0d284b5445de5243a8931f
Partial-Bug: 1254555
Merge "Rebind allowed address pairs only if they changed"
Merge "Add test_handle_router_snat_rules_add_rules"
Merge "Do not add DHCP info to subnet if DHCP is disabled"
Merge "Fix random unit-test failure for NVP advanced plugin"
Merge "Avoid loading policy when processing rpc requests"