Fix loading the Galaxy dependency resolvers config, plus additional config directory fixes

[natefoo]

Updates to galaxy imports a while back broke loading the dependency resolvers config file (when using galaxy-tool-util instead of galaxy-lib).

Changes:

• The dependency resolvers config file is now passed to the dependency manager builder
• Pulsar no longer changes directories to $PULSAR_ROOT on startup unless you were to call python ./pulsar/main.py, as under any other circumstances you are working with an "installed" Pulsar (even if using pip install -e /path/to/pulsar-clone). This was causing a lot of mess in /path/to/pulsar-venv/lib/pythonX.Y/pulsar for me in the past.
• Dependency resolvers config and job metrics config now searched for in the same dir as app.yml if they're not absolute.
• config_dir is now passed into the app constructor instead of passed in as dir due to PULSAR_CONFIG_* env var handling.

I also tested that you can set the resolvers under dependency_resolution.resolvers like so in app.yml:

dependency_resoution:
  resolvers:
    - type: conda
      auto_init: false
      auto_install: true
      prefix: /path/to/conda

This should(?) probably be the preferred method going forward once we update documentation. This also works under the galaxy key of galaxy.yml in Galaxy.

Relatedly, the behavior here also looks wrong, it shouldn't be returning job_metrics from the kwargs if passed in. But I think galaxy.job_metrics:JobMetrics doesn't load config from a dict yet anyway.