sos2: fix callExtProg usage

fix usage of callExtProg method causing sos plugin
to exit due to a TypeError exception.

Change-Id: I02574b2d4ac35bac137db82060ccc18b5d27a372
Bug-Url: https://bugzilla.redhat.com/1123808
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>