ConfigFile.py: test for key=val pattern only after replacing old lines.

Only after changing an old line (by removing or adding a prefix to it)
should we consider the line as an old key=val pair - We do not replace
old configuration keys if they exist in a configuration file.

Change-Id: I3cb6094d8a1feeb45630d29e36ecad2ddfac1fca
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27823
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>