Add VIRL host selection mechanism to bootstrap

Add a mechanism to the bootstrap script that allows it to select
one out of multiple candidate VIRL hosts. In addition, fix a few
cosmetic issues wrt. connecting to the VIRL host:
* Use consistent set of SSH options
* Consistently use constant instead of hardcoded key filename
* Use relative path names for start-/stop-testcase scripts

Change-Id: I0d2972f170de82dd5d98da88656f5f962fca2415
Signed-off-by: Carsten Koester <ckoester@cisco.com>