nova coverage creates lots of empty folders

With nova coverage extension disabled as a default behaviour,
nova will create lots of empty folders under /tmp folder. And also
with each time the nova service reboot, it will create a new
directory with name like nova_coverage_*. There will be number of
the empty folders accumulated.

Patch-Set 3: Rebase for merging code
Fixes: bug 1169005

Change-Id: I26d1037eb66d781ac6ef6b600cca04a709bff3f7
Merge "nova coverage creates lots of empty folders"