install.py:  Warn if purging all data and ceph still installed.

If any of the hosts listed in the purgedata command still have ceph installed
issue a warning message and prompt to continue. Fixes #5300.

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>
Reviewed-by: Alfredo Deza <alfredo.deza@inktank.com>