Merlin

Fixed
- Fatal bug triggered by a spec missing the `env` or `global.parameters` sections.

Added
- When using the `--samplesfile` flag, the samples file is now copied to `merlin_info` for
provenance.

Fixed
- Exceptions in connection check sub-process will now be caught.

Added
- The ability to override any value of the celery configuration thru `app.yaml` in `celery.override`.
- Support and faq entry for `pgen` with `merlin run --pgen` and optional `--parg`.
- Documentation on `level_max_dirs`.
- Easier-to-read provenance specs.
- Documentation on the new 3 types of provenance spec.

Fixed
- Flux test example data collection for new versions of flux.
- Fixed Docker ubuntu version.
- Removed expansion of env variables in shell sections (`cmd` and `restart`) of provenance
specs. This allows the shell command itself to expand environment variables, and gives
users greater flexibility.
- Allowed environment variables to be properly expanded in study `description.name`.
- Tilde (~) now properly expands as part of a path in non-shell sections.
- The rediss cert_reqs keyword was changed to ssl_cert_reqs.

Changed
- Updated tutorial redis version to 6.0.5.

Added
- The sample generation command now logs `stdout`, `stderr`, and `cmd.sh` to `merlin_info/`.
- 12 hidden test specs and associated cli tests, for cli tests with specs that we
do not want in `merlin examples`.
- Inside `merlin_info/`, added provenance specs `<name>.orig.yaml`, `<name>.expanded.yaml`, and
`<name>.partial.yaml` (identical to the original spec, but with expanded user variables).

Fixed
- Updated to new celery (4.4.5) syntax for signature return codes.
- Corrected prior visibility timeout bugfix.
- Fixed and reactivated 3 cli tests.
- Added the `bank` and `walltime` keywords to the batch slurm launch, these
will not alter the lsf launch.

Changed
- Slightly improved logic by using regex to match variable tokens.
- Reduced instances of I/O, `MerlinStudy` logic is now in-memory to a greater extent.

Fixed
- Error if app.yaml does not have visibility timeout seconds.

Added
- The broker name can now be amqps (with ssl) or amqp (without ssl).
- The encryption key will now be created when running merlin config.
- The merlin info connection check will now enforce a minute timeout
check for the server connections.

Fixed
- Added a check for initial running workers when using merlin monitor to
eliminate race condition.
- A bug that did not change the filename of the output workspace nor of the provenance spec
when a user variable was included in the `description.name` field.
- Temporarily locked Celery version at 4.4.2 to avoid fatal bug.

Changed
- The default rabbitmq vhost is now <user> instead of /<user>.
- Changed default visibility timeout from 2 hours to 24 hours. Exposed this in the config
file.
- The monitor function will now check the queues to determine when
to exit.