sos: add sos 3.1 support

- conditional install sos plugins based on sos version
- use sos-3 plugins if available
- requires sos > 3.1 on RHEL >= 7 and Fedora >= 20;
  plugins have been moved on sos upstream.

Change-Id: I47fb7817727ed88a1b9c7bb91cb5055a2e8d7565
Bug-Url: https://bugzilla.redhat.com/1037663
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>