install: need sudo when purging /var/lib/ceph

Signed-off-by: Sage Weil <sage@inktank.com>
verify /var/lib/ceph not present on start

Verify there is no /var/lib/ceph, just like we do with the cephtest
directory.  We will need to change this (or make it optional) when we
allow runs against an existing cluster, but then a whole bunch of other
things will need to change then as well.

Signed-off-by: Sage Weil <sage@inktank.com>