Allows to upgrade the cache using the sssctl tool, which might be useful
e.g. in RPM %post scripts.

Since we want to use the sssctl tool during upgrade, we need to amend
the tools initialization code to not error out if sysdb can't be
instantiated, but rather return errno and let the tool handle the error.

Each tool command now has a 'allowed errno' the command is able to
handle. In this patch iteration, only a single errno can be handled and
only the upgrade command is able to do so.

For users and groups, convert the input name to the qualified format.

Resolves:
    https://fedorahosted.org/sssd/ticket/3059

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>