generic.tests: Make the guest status collect configurable for netperf

Now the guest statys collect functions are implemented and tested with
RHEL and Fedora guests. It may cause problem for other guests. So set
it default to no, only enable it for RHEL and Fedora guests.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #95 from ypu/netperf

generic.tests: Make the guest status collect configurable for netperf