webadmin: Lift max MTU limit

Don't limit users to a maximum MTU value when configuring a network,
values too high will cause errors when provisioned on a non-supporting
host. Also removed the reference to the MaxMTU configuration value
which had never been exposed to users anyway.

Change-Id: I352ac3e00d657f00b6cec5dcb23475a143a9c8fd
Bug-Url: https://bugzilla.redhat.com/1010663
Signed-off-by: Lior Vernia <lvernia@redhat.com>