cover.sh: delete .testrepository before running coverage

cover.sh: delete .testrepository before running coverage

When running py27 and py3 test ran into problems
with .testrepository already exists, but testr
thinking it was corrupt

running testr
No repository found in /home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master. Create one by running "testr init".
error: testr failed (3)

The fix seems to be to delete .testrepository before
running testr coverage

Change-Id: Ib8cd3ab9d3384935380ac29ce365439c6464adc3
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>