XD-1972: Add ability to define nested jobs
JIRA: https://jira.spring.io/browse/XD-1972
* add suport for nested jobs
XD-1972: Made nested jobs a non-breaking change

This commit converts the logic of how a job is named so that if there is
more than one Job in the module's context, the one to be executed must
have the id "job".  If only one Job exists in the module's context, then
it can be any name.
XD-2226 ZooKeeper namespace is now configurable

The zk.namespace property can be set in servers.yml (default is "xd").