admin/pin: Add commands to pin booted, pending and rollback deployments

admin/pin: Add commands to pin booted, pending and rollback deployments

Add new commands to pin the current, staged and previous deployment for
use in automation and scripting. Right now, it's difficult to pin the
current deployment without needing to look into the output of some other
tooling (like rpm-ostree) to get the index of each deployment. This
index also is not consistent - the current deployment could be 0 when
you first boot the system then 1 shortly after. This change makes it
easy to pin the current or future deployment.

Co-authored-by: Robert Sturla <robertsturla@outlook.com>
Signed-off-by: Eric Curtin <ecurtin@redhat.com>