Add block store troubleshoot docs

This patch adds block storage troubleshooting documentation,
which superseeds the old documentation.

It's actually https://review.openstack.org/#/c/49972 - but that
patch became a rebase hell, hence the new one.

Change-Id: I37621e1bede368b57a5af5e33b2d131c7e28d488
Merge "Add block store troubleshoot docs"