webadmin: Host Edit Network dialog - fix custom properties layout

Make dropdown wider so whole string 'Please select a key...' is visible.

Change-Id: Iaf25dde046346f8bea239b9e3f6c963f86e74adc
Bug-Url: https://bugzilla.redhat.com/1124404
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>