Handle transitions between BatchAMI versions

[dpwrussell]

AWS Batch currently has no mechanism to add custom initialisation when provisioning instances. This feature does exist in ECS, so it seems likely it will eventually be added to Batch as well. Once this is possible it will be possible to dispense entirely with the custom AMI.

Until that time, the custom AMI (which is preconfigured for the specific EFS volume) is required. To upgrade from one AMI to another it will be necessary to delete and create new EC2/spot compute environments. This can't be done while jobs are active. An approach which adds the new compute environments, switches over to them and them removed the old one once it was drained would be logical.

Note: To avoid building an AMI for each Minerva deployment it would have been nice to be able to use environment variables to inform each instance what EFS volume to mount, but unfortunately there is no mechanism to supply the Batch instances with environment variables either.