vswitchpef: avoid build at ericsson-build3

VSPERF's VERIFY & MERGE jobs are having issues at ericsson-build3
since September. The problem is, that after a few successful runs,
machine starts to be very slow and thus it is not possible to execute
neither OVS nor VPP anymore. Debugging together with responsible
admins didn't help to find a root cause (probably a race condition
during execution of vsperf & other projects). As a temporary measure,
I want to tight vsperf jobs to ericsson-build4 only, where they work
without issues.

Change-Id: I62e482f57234f90fc127b6e6d9454488e17657fc
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>