Avoid initialization of policy engine before extensions are loaded

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