Revert "Filtering routers by network-id"

It was pointed out on spec [1] that we should not follow
a piece-meal approach in extending filtering capabilies
to the API and we should have a holistic approach to this
during the API refactoring work.

Work [1] was therefore deferred, and this one should be to.

[1] https://review.openstack.org/#/c/138695/

This reverts commit bf80173046646eb1e829574bd9668cd1a84745b8.

Change-Id: Ic34966c78e684fd3d764d1e941c5a318dffd1932
Merge "Revert "Filtering routers by network-id""