Make netconsole able to be disabled

Previously, we weren't able to unset netconsole parameters, since
augeas cannot reliably comment files, and setting values to None
put netconsole in a bad state. Resolve this.

Change-Id: I54f3c2123408462d20f7df3366933160d9a4fc28
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1051434
Signed-off-by: Ryan Barry <rbarry@redhat.com>