test: set a default $CEPH_ROOT env variable

test: set a default $CEPH_ROOT env variable

in cf24535, we use $CEPH_ROOT to specify the $top_srcdir to unify
cmake and autotools, but this breaks ceph-qa-suite/tasks/workunit.py,
as it only clones the necessary qa/workunits directory, and does not
pass $CEPH_ROOT to the test scripts. so we need to set a default
$CEPH_ROOT if it is not set.

Signed-off-by: Kefu Chai <kchai@redhat.com>