vnfs: configurable loopback application support inside VM

vnfs: configurable loopback application support inside VM

For PVP and PVVP deployments, traffic inside virtual machines must be
forwarded between its interfaces. There are several forwarding options
supported by VSPERF, including DPDK testpmd, Huawei l2fwd kernel module
and linux bridge. Another option is, that VM image has built in forwarding
functionality and vsperf doesn't configure it itself.
User can select VM loopback application by configuration option
GUEST_LOOPBACK (per VM) or by SCALAR cli option guest_loopback (global
settings).
Selected guest loopback application(s) will be written into test results
CSV file and test report MD file.
Default wildcarded guest login prompt was added. Guest startup guarding
timer can be configured. Path to OVS kernel module has been fixed.

Change-Id: If738da1ea09112f9cf2267afcbc99a6797f3a03a
JIRA: VSPERF-130
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>