Externalize data source auto configuration to allow easier migration to latest SpringBoot

Currently data sources are directly auto configured in JBPMAutoConfiguration which tightly couple the jBPM with data sources and transaction manager setup. Data sources should be externalised to allow switch of different versions of data sources setup to allow both easier upgrade to spring boot 2.x and to allow to use latest versions of jBPM to run with both spring boot 2.x and 1.5.x