XD-1972: Add ability to define nested jobs

JIRA: https://jira.spring.io/browse/XD-1972
* add suport for nested jobs
I realize this is probably a bit late in the game, but I'm wondering if there is a way to address this without making it a breaking change.  As it's coded right now, all jobs going forward need to have the job's id = job.  I'm wondering if we can be a bit smarter so that that restriction only applies when there is more than one job defined in the context.  For example, here we could check to see if this is the only bean of type FlowJob in the context.  If it is, don't worry about the job name.  Thoughts?