BugLink: http://bugs.launchpad.net/bugs/1659417

There is no longer any need to raise additional capabilities
in the mounter's credentials, so we can do away with the extra
complexity of cloning the credentials and associated error
handling. Replace that with the same interface which was later
adopted upstream, ovl_override_creds().
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>