Support diverse setup disks

This brings the support of heterogeneous hardware. Not all the servers
are identical, some have more or less disks than the others. Prior this
commit, the 'parted' command was hanging, now the command simply exits 1
if the device doesn't exist, same for the 'egrep' piped command after.
Then we skip these errors and continue to run. So now, you can specify
multiple devices in group_vars/osds that don't exist on all the
servers.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>