packaging: engine-backup: Safer password input

Allow passing db password also interactively and using an environment
variable, in addition to using a file and using a command line option.

Bug-Url: https://bugzilla.redhat.com/1002401
Change-Id: Icbc497af2653417beab54a9be307332cdf69a48a
Signed-off-by: Yedidyah Bar David <didi@redhat.com>