libcephfs: add ceph_conf_parse_env()

This exists in the librados API.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_libcephfs: parse environment

Lets you use CEPH_ARGS to get output from the tester.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #279 from ceph/wip-libcephfs-env

Reviewed-by: Greg Farnum <greg@inktank.com>