Streamline launching new nodes.

* launch/README: More clarity on Jenkins slave example, and
additional levels of cut-n-pasteability on the DNS record creation
example. Also switch from requiring root to expecting to be run from
a normal account with sudo access and membership in the puppet
group.

* launch/launch-node.py: Default to assuming the certname is the
same as the node FQDN, if it isn't overridden via command-line
option.

Change-Id: I9c987055b18e084983f2459fe01598837e1ebcc6
Reviewed-on: https://review.openstack.org/20645
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins