test: cleanup should clean all left over (stale) mounts

This is a temporary work around to fix the spurious failures seen in ec
testcases. As per the initial analysis it looks like quota
(glusterd_quota_initiate_fs_crawl) is causing a mount point in /tmp to be stale.
Once the root cause is identified this fix can be reverted as well.

Backport of http://review.gluster.org/#/c/8703/

Change-Id: I8686f144ed298124074f749e75c13028ec00be01
BUG: 1141187
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/8703
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Justin Clift <justin@gluster.org>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8708