Add "Edit Router" to allow to change router type

Neutron DVR implementation allows to change router type from
centralized to distributed. This commit adds "Edit Router" form
which is not implemented so far to allow this feature.

This commit also adds:
- admin_state field to the router detail.
- documentation on a new option enable_distributed_router

Completes blueprint enhance-horizon-for-dvr
Change-Id: I4b46e44c417726217ed034e305827b102ba656f8
Merge "Add "Edit Router" to allow to change router type"