webadmin: Made interface mappings validation less space-tolerant

It used to accept whitespaces, but it would interpret them in a
non-intuitive way; they would be considered part of the
label/interface/bridge name.

Change-Id: I5a2240dda423b026fbaf99613cfba89c9b3a1fd7
Signed-off-by: Lior Vernia <lvernia@redhat.com>