webadmin: Add some spacing to checkbox cell labels

Some regression occurred in 3.5 where labels in checkbox cells too
close to the actual checkbox (presumably due to the Look & Feel
patches). This adds some space between them.

Change-Id: I2e345fcfdfb18c8305598e0affc87c2e6c196092
Bug-Url: https://bugzilla.redhat.com/1120984
Signed-off-by: Lior Vernia <lvernia@redhat.com>