CPERF: Fixes issue with known hosts

Exporting the global var to disable ansible host key checking doesn't
ignore known hosts in the file. To fix this, this patch sets the known
hosts file to /dev/null.

Reference: https://github.com/ansible/ansible/issues/9442

Jobs currently failing due to known hosts:
https://build.opnfv.org/ci/job/cperf-apex-csit-master/320/console

Change-Id: Ic3470b368a056b3a3981f9555160a44018f97ebd
Signed-off-by: Tim Rozet <trozet@redhat.com>