Grab-bag improvements

[johnmaruska]

I let this go way too long before merging back, ugh. Do not do this.

Allow caller to specify machine memory for compute instance

• start.sh accepts --memory-gb to specify instance memory
• start.sh generates a cromwell.service file with correct mem
• start.sh accepts a --tmp-dir to store generated cromwell.service
• remove cromwell.service from Dockerfile since it's generated

Cromwell resource sizings

• double server VM resources
• allow db instance to be specified as a var

Miscellaneous improvements

• default to preemptible for Bolton and Griffith labs
• correct exceptions in previously untested paths (json.dump, dict.get())
• compatibility changes to start.sh to work with other sh implementations
• pull_outputs skips outputs already downloaded
• pull_outputs specifies its outputs directory properly when workflow_id is included
• submit_workflow.sh is part of Dockerfile for convenience
• persist cromwell.log from server
• GMS requires additional service account binding