dm vdo: add dmsetup message for returning configuration info

Add a new dmsetup message called config, which will return
useful configuration information for the vdo volume and
the uds index associated with it. The output is a YAML
string, and contains a version number to allow future
additions to the content.

Signed-off-by: Bruce Johnston <bjohnsto@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>