improve(engine): add specific exception for mismatching correlation

- throw in case more than one or no execution/process definition
is correlated

contributes to CAM-664
chore(tasklist): fix engine setup for development
chore(tasklist): fix form in dev setup
chore(rest): remove task counts from group info method

- task counts do not belong to an identity resource
- not needed in tasklist

closes CAM-660
style(rest): remove unused imports
fix(rest): fix and test engine-specific identity rest methods

contributes to CAM-653
fix(engine-history) history level missing

- BUGFIX: fix problem with no history level written in db after creation of engine tables by user and property 'databaseSchemaUpdate' set to 'true'. Added check which tests for existance of historyLevel property in 'property'-table and inserts it when not set.
- REFACTOR: refactored failing tests in testsuite after history level fix

related to #CAM-571
Merge remote-tracking branch 'refs/remotes/origin/master'
fix(engine-integration): remove engine from spring test deployment
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
fix(tasklist it): fix label change
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
fix(engine-history) history level missing

- BUGFIX: Test suite failed when Jenkins runs tests in random order

related to #CAM-571
fix(engine-history) history level missing

- BUGFIX: Test suite failed when Jenkins runs tests in random order

related to #CAM-571
fix(history): Disable checkHistoryLevel
fix(renderer): Fix line break in renderer
 Prepare release: set version to 7.0.0-alpha3
 Next development version: 7.0.0-SNAPSHOT
feat(rest): add message correlation method

- POST /message with json request body
- translates to RuntimeService#correlateMessage

closes CAM-695, contributes to CAM-636
feat(rest): make message servive avaialable per engine

- /engine/{name}/message

closes CAM-696