calamari_setup: mounting iso on older distros requires -o loop

Ubuntu's mount/kernel support "mount <file> <mntpnt>" directly;
apparently Centos 6 (and presumably RHEL6) require specifying at
least '-o loop' (a /dev/loopN will be dynamically allocated and removed
on unmount).

Signed-off-by: Dan Mick <dan.mick@redhat.com>

tasks/calamari_setup.py

@@ -294,7 +294,8 @@ def calamari_install(config, cal_svr):
     elif icetype == 'iso':
         mountpoint = '/mnt/'   # XXX create?
         ret = cal_svr.run(
-            args=['sudo', 'mount', '-r', remote_iceball_file, mountpoint]
+            args=['sudo', 'mount', '-o', 'loop', '-r',
+                  remote_iceball_file, mountpoint]
         )
 
     # install ice_setup package