Rework Spack's continuous integration workflow to be environment-based.
- Add the `spack ci` command, which replaces the many scripts in `bin/`
- `spack ci` decouples the CI workflow from the spack instance:
- CI is defined in a spack environment
- environment is in its own (single) git repository, separate from Spack
- spack instance used to run the pipeline is up to the user
- A new `gitlab-ci` section in environments allows users to configure how
specs in the environment should be mapped to runners
- Compilers can be bootstrapped in the new pipeline workflow
- Add extensive documentation on pipelines (see `pipelines.rst` for further details)
- Add extensive tests for pipeline code
- Add a `spack gpg` subcommand in anticipation of signed binaries.
- GPG keys are stored in var/spack/gpg, and the spack gpg command manages them.
- Docs are included on the command.
