sos: engine: don't collect db passwords

obfuscate sensitive keys in collected configs.
avoid to collect legacy empty files.

Change-Id: I169d47db602584093210bc4795ff64abc22824fc
Bug-Url: https://bugzilla.redhat.com/1035826
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>
(cherry picked from commit 2122d59b43af58bf72f2e7bc2b614735f9362f24)