global: update HOME environment variable when dropping privileges

global: update HOME environment variable when dropping privileges

k8s/rook is currently starting daemon pods under root using the
"--setuser" CLI optional to drop priviledges. In the case of
rbd-mirror which creates connections to remote clusters via librados,
the default search path for Ceph config files includes
"$home/.ceph/$cluster.conf", which before this change would evaluate
to "/root/.ceph/..." and then fail with a -EPERM since that directory
is not accessible by the dropped priviledges user.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 591fb2bf686be770fc07d7b02d54c4ccff81620f)

Conflicts:
	src/global/global_init.cc
- master has g_conf()-> where luminous does g_conf->