Older versions of swauth supported manually setting up a salt string in
conf file. This change re-introduces it and makes it a tunable option.
The current behavior of randomly generating salt for every password is
NOT affected with this change.
Change-Id: Ifdf6f806b954e4d41c083eeffa981cd7d0dd50b9
Signed-off-by: Prashanth Pai <ppai@redhat.com>