Fix context problem

Fix context problem

On plugin side, we use normal API context if any to call RPC methods.
We use our plugin dispatcher to convert RPC context into quantum admin
context. After that the callback's functions have the first argument context
as normal quantum context.

On agent side, we use admin context without session property as its
RPC calling context. Call back context is default RPCCommonContext in
Openstack common modules.

This patch also fixes the problem in the following bug:

Bug #1077012

Change-Id: I913b48dcd84d275cd7de30ca990be00c243e63ea