nuke: tolerate failed dpkg --configure -a/apt-get -f install

Signed-off-by: Sage Weil <sage@inktank.com>
rgw: specify keyring location

Otherwise we look at the default /var/lib/ceph/radosgw/ceph-$id/keyring.

Signed-off-by: Sage Weil <sage@inktank.com>