Avoid cinder calls, when cinder is unavailable

When a volume service is not available, then a cinder client can not
be created. This patch skips the calls, so that dashboard doesn't
break any more.

This is a backport to Folsom release, as this turned out to be a major
issue.
Fixes bug 1084137

Change-Id: I8f2f8b0b131b4bb5319d74f6da48671f146d7e00
Merge "Avoid cinder calls, when cinder is unavailable" into stable/folsom