l3-agent: move check if ext-net bridge exists within daemon loop

l3-agent: move check if ext-net bridge exists within daemon loop

bug 1052522

the l3 agent checked if the external network bridge exists in its
constructor, raising an uncaught exception if it did not.  this does not
make much sense when running the l3-agent as a deamon, especially since
it can be the case that the l3-agent starts before open vswitch.

Change-Id: Ie1717b2c02c9f0bc0caf34a6fdb0dc3a930123c0