reana-admin: implement command to migrate secret key

[mdonadoni]

REANA needs a command to change the secret key used to encrypt database columns. The columns need to be decrypted with the old key and re-encrypted with the new one.

See https://github.com/inveniosoftware/invenio-db/blob/76014ba16d47790f49738ea255525db236755151/invenio_db/utils.py#L21 and https://github.com/inveniosoftware/invenio-base/blob/8c7235b3ba9fb1fbdb19642b28b9b7d28ed9bc41/invenio_base/cli.py#L57