common/util: pass real hostname when running in kubernetes/rook container

common/util: pass real hostname when running in kubernetes/rook container

Rook will set up some environment variables to tell us what pod, pod
namespace, and physical host we are running on.  If we see them, set
'hostname' according to the hardware host, and include the pod info
in our metadata.

This allows the usual host-based logic on Ceph to work correctly, like
placing OSDs automatically in the CRUSH map.

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