Support migrating of legacy routers to HA and back

Support migrating of legacy routers to HA and back

This patch adds support for migration of legacy routers to HA and
vice-versa. This patch also:

1. Reverts I4171ab481e3943e0110bd9a300d965bbebe44871, which was used to
   disable such migrations until support was inserted to the codebase.
2. Adds an exception to indicate that such migrations are only available
   on routers that have their admin_state_up set to False.

Closes-Bug: #1365426
DocImpact (Handled in patch 233695)
Change-Id: Ie92f8033f47e1bf9ba6310373b3bfc9833317580