webadmin: Allow setting MTU > 9000

Fetch the max MTU value from the database (as is done in the backend)
instead of hard-coding it to be 9000.

Change-Id: Ie427c578d79df6c37faac4ef80e697566b8e9fb3
Bug-Url: https://bugzilla.redhat.com/1010663
Signed-off-by: Lior Vernia <lvernia@redhat.com>