Fix router listing under OVS

Override methods related to finding routers for
particular agents to always return all of the
routers, since we don't *have* an agent.

Change-Id: I2bf08392fe291265fea21d53b27d9027dd29da48
fix pep8 error

Change-Id: If70982c0fa68a61d70958fd8957739226582e5f3
simplify list_routers_on_l3_agent

Change-Id: I3574b977604faedc33699a4154d20e18ad11b963
restore filter to router listing

Filter when given a list of router ids.

Change-Id: I68db050a0de8b26ac1fcd6a018690c0e32554677
simplify logic in filter handling

Change-Id: I8ee0ac867529e0b6750d97dee1576958fe4f20b4
Merge pull request #31 from dhellmann/fix-ovs-router-listing

Fix router listing under OVS