Changes

jenkins-common.sh: exit early on full disk

When my disk runs full, jenkins-common.sh fails to create a VOL_BASEDIR right
at the start, and hence no testsuite gets its external volumes mounted
properly. However, the test suite still starts up with missing cfg files etc,
and fails in non-obvious ways, until I find out the disk is full some minutes
later. Instead, verify that the base dir really exists or bail out right there.

Change-Id: I6540797c8e0ee3e7b09d4a80592d5e270e7d9adc