vdsm-tool: add unprefixLines() to configfile.py.

New method removes 'prefix' from each line starting with it.
No editing is done on new content added by this config file.

This method will be used to remove '# VDSM backup' comments
put inside libvirt logrotate file.

Changed doc string of prefixLines to reflect the link
between the two methods.

Tests included (see testPrefixIdempotencey in toolTests.py).

unprefixLines is the reverse function of prefixLines so that
unprefixLines(prefixLines(file)) = file.

Change-Id: Idd869dced51f1e67c9fb1bb264424b3758cc54f0
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27737
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>