use the right username when computing the db password hash

[evgeni]

some people use other names than pulp for their DB user

[wbclark]

Interesting, how did this come up?

Probably no point to add Refs #28901 now given how old that feature is. Is there some newer associated Redmine?

[evgeni]

Interesting, how did this come up?

user had external db deployed with pulpcore as the username, now they are moving back to internal and when setting manage=true they ended up with not being able to talk to the db anymore.

[evgeni]

Oh, and to add a pinch of mystery to this: You need to use MD5-auth (so Foreman 3.10 or older) to actually run into a problem here.
With SCRAM the code here would still use the "wrong" salt, but that salt is properly stored in PostgreSQL (and can actually be anything!) and thus authentication works.