Using the current working directory as the base test directory
fails with an unclear error message when running mkfs for OSDs
if tests are run when the k8s master node is not running on
localhost.

This change creates a new flag for the test suite to set the
base test directory, but uses a default that is safe for
virtualized test environments ("/data").

Fixes: #4205
Signed-off-by: egafford <egafford@redhat.com>