webadmin: Add Interface dialog is cropped in chrome.

The default width of the inner element of mainPanel wasn't big enough.
It caused some elements to break into two lines,
it caused the bottom elements of the panel to be cropped.

Change-Id: If0e537c9ada3bcc3a0b108228442b59eda9fe26f
Bug-Url: https://bugzilla.redhat.com/905514
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>