Rewrite regular expression to avoid salt-minon exception

If ceph-fuse is deployed, a socket named "ceph-client.admin.asok" would appear under
`/var/run/ceph`, and cause current code AttributeError. So this patch rewrites the regular
expression and adds some fault tolerance code. Also note that the first two groups of the
regular expression are made non-greedy to parse names like "ceph-mon.www-2.abc.com.asok"
correctly.