Idea: run a specific init job

[igobranco]

Currently or you run all init jobs or you run any, using the DRYDOCK_INIT_JOBS variable.
But when you have a complex system, probably the operator needs to run/init only partial of the services.
For example if you have Open edX + Richie, you could only want to run DB migrations only for Richie and not for all Open edX services.

Ideally, IMO there could be another variable that DRYDOCK_INIT_JOBS_LIMIT with a list of all services that the operator wants to have the job/init to run.