User passwords will be encrypted with SHA-256

Changes in UserImpl.java also cover situations when an authenticating
user still has an MD5 password set. In this situation, as long as
the user authenticates successfully, we encrypt the password with
SHA-256 and store it in the DB (i.e. we replace the old password).
Support SHA-256 encrypted user passwords in backend

Whenever a user with an MD5 encrypted password authenticates
successfully, backend will encode its good password with SHA-256
and store it in the DB.
RPC session hash: md5 -> sha256

We cannot use MD5 for a FIPS enabled Spacewalk.