ceph: respect the PYTHON environment variable for dev mode.

On OSes where `python` is python3, dev mode's re-exec makes the
ceph tool fail. The standard way to fix this is by exporting
the PYTHON envvar pointing to the python2 interpreter.

Signed-off-by: Dave Anderson <dave@natulte.net>
Merge pull request #3103 from danderson/ceph-dev-mode-python-envvar

ceph: respect the PYTHON environment variable for dev mode.

Reviewed-by: Loic Dachary <ldachary@redhat.com>