Add the agent extension alias to Midonet plugin.

* Add the agent extension alias to the Midonet plugin.
* This alias is needed to support the listing of network agents.

Signed-off-by: Joe Mills <joe@midokura.com>

Fixes MN-1210
Merge pull request #90 from JoeMido/MN-1210

Add the agent extension alias to Midonet plugin.