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

I removed the unecessary plugins from the build and reworked it to use
the ci friendly way of setting the version for releases (see
https://maven.apache.org/maven-ci-friendly.html).

This removes the spring-boot-starter-parent from the build, and
duplicates the necessary plugin configuration in he Spring Boot Admin'
parent instead. This way our BOM doesn't include version for
dependencies managed by boot's BOM

The info should be updated when the registration has changed. In order
to prevent a stale instance info, the sba client includes a "startup"
timestamp in the metadata as default. So when the instance is restarted
the registration changes and the info is updated, even when the instance
isn't considered as down in between.

To prevent this when using a discovery client you need to add a value
to the metadata which changes with every restart.
(e.g. `random=${random.value}`)

fixes #556

Don't output metadata in Registration.toString()