In order to do a relaease not waiting for Spring Cloud
Finchley.REALEASE move all depedent stuff into a seperate module which
can be excluded during build

- use MimeMessage with Content-Type text/hmtl
- use a single template for subject and body
- use a Spring aware TemplateResolver

This commit enables client and server to work out of the box on
CloudFoundry.
The client derives the registration info (url and metadata) from the
VCAP environment variables.
The server computes a unique instanceId and the correct headers for the
cf router based on the metadata provided by instances. Also suppots the
CloudFoundry discovery client ootb.

closes #434

... to support multiple HttpHeadersProviders

fixes #647

closes #560

It turns out that imho when talking about application groups (identified
by name) and application instances it's just logical to speak of
instances and applications. So the Application class get's renamed to
Instance. So an Application is now a set of Instances with the same name

To avoid some strange config sorting errors using the
DeferredImportSelector, switch to a real AutoConfiguration activated by
a marker bean

The actuator endpoints are detected by either querying the index
endpoint for spring boot 2.x applications or by probing a configurable
list of endpoints using a OPTIONS request.

* extract InfoUpdater from StatusUpdater
* rename ApplicationJournal to ClientApplicationEventStore
* ClientApplicationEventStore now implements reactor's Publisher
* Reworked AdminApplicationTest to be more sophisticated.