ceph-handler: Use /proc/net/unix for rgw socket

ceph-handler: Use /proc/net/unix for rgw socket

If for some reason, there's an old rgw socket file present in the
/var/run/ceph/ directory then the test command could fail with

test: xxxxxxxxx.asok: binary operator expected

$ ls -hl /var/run/ceph/
total 0
srwxr-xr-x. ceph-client.rgw.rgw0.rgw0.68.94153614631472.asok
srwxr-xr-x. ceph-client.rgw.rgw0.rgw0.68.94240997655088.asok

We can check the radosgw socket in /proc/net/unix to avoid using wildcard
in the socket name.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>