release workflow: Add spack command for generating the .gitlab-ci.yml

This spack command adds a new schema for a file which describes the
builder containers available, along with the compilers availabe on
each builder.  The release-jobs command then generates the .gitlab-ci.yml
file by first expanding the release spec set, concretizing each spec
(in an appropriate docker container if --this-machine-only argument is
not provided on command line), and then combining and staging all the
concrete specs as jobs to be run by gitlab.