Forbid update of HA property of routers

While the HA property is update-able, and resulting router-get
invocations suggest that the router is HA, the migration
itself fails on the agent. This is deceiving and confusing
and should be blocked until the migration itself is fixed
in a future patch.

Change-Id: I4171ab481e3943e0110bd9a300d965bbebe44871
Related-Bug: #1365426
Closes-Bug: #1378525
Merge "Forbid update of HA property of routers"