1165355 - Sanitize checksum types.

SHA and SHA-1 are the same checksum algorithm, but Pulp considered them
to be separate algorithms. This commit ensures that Pulp no longer
stores the SHA-1 algorithm as "sha" in the database, and it also
contains a migration that resolves existing "sha" objects.

https://bugzilla.redhat.com/show_bug.cgi?id=1165355