Currently the DNS search domain value is validated in the backend.
But this validation takes place at the time of instance creation,
since that is when the static edge bindings are created and pushed
to the backend. This allows the subnet create/update operations to
succeed even if an invalid DNS search domain value is specified.
This patch adds validation to the DNS search domain extension.
Change-Id: Ib392e8695f40023219df93b6889366ce0a305423
4c00489dbb