main: Make ostree --version output YAML (and add gitrev)

main: Make ostree --version output YAML (and add gitrev)

I learned today that `docker version` does this and I really like
the idea.  While we have the patient open, also add the gitrev
with code taken from https://github.com/projectatomic/rpm-ostree/pull/584

Closes: #691
Approved by: giuseppe