Avoid initialization of policy engine before extensions are loaded

Bug 1174877

For a particular NVP extension, a database entry was being populated
in the plugin's __init__ method. The entry was added with an admin
context. Grabbing the context caused the policy engine to load before
all the extension were loaded, so rule on extended attribute
were not being parsed correctly.
This cause checks on external networks to fail always.

Change-Id: Icbc759773acab2a7687883d8d2844272c8ea55a3
Set network_device_mtu as integer

Fixes bug 1175060

Change-Id: Ibce28db0575dd6d6f1f998a7b8d6603f3d6dba63
Allow admin to delete default security groups

Previously there was no way to delete a default security groups which
isn't ideal if you want to clean up after deleting a tenant. This patch
allows default security groups to be deleted by the admin.

Fixes bug 1175393

Change-Id: I2214c7dabf0f2ec960ce10ebbbcdc513bc73664c
Merge "Avoid initialization of policy engine before extensions are loaded"
Merge "Set network_device_mtu as integer"
Merge "Allow admin to delete default security groups"