Allow L3 base to handle extensions on router creation

Allow L3 base to handle extensions on router creation

By changing the boolean flag, API extensions made to
the router model can be handled correctly: this means
that on router creation, the response body will
contain all the extension attributes being part of
the resource. Prior to this fix, it was only on GETs
or PUTs, leaving the user at loss as to whether
the flag was actually being processed.

Closes-bug: #1325608
Supports-blueprint: neutron-ovs-dvr

Change-Id: I6f913c8417676a789177e00f30eb5875e7aaa3ae