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

When running on any CloudPlatform the auto-deregistration defaults to
true otherwise the behaviour is unchanged.

closes #658

In order to make the starters compliant to the spring boot naming rules
I added a codecentric-* prefix to the server and the client starters.
All samples are now using our starters and the docs were alignes with
these changes. In order to not suprise existing users I've added a
relocation to our old module

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

fixes #590

fixes #515.

Since there is no Heartbeat or InstanceRegisteredEvent when using the
SimpleDiscoveryClient we need to listen on the ApplicationReadyEvent
and register the configured instances

closes #505

The Spring team discourages the use of the `spring.*` configuration
namespace, hence I use the major update and move all properties. With
this commit all client related properties are moved to `sbadmin.client`.

With this commit two new properties were added to ease the client
configuration when used in servlet containers:
`spring.boot.admin.client.management-base-url` and
`spring.boot.admin.client.service-base-url`. Both properties can be used
to specify protocol, host and port for the urls. The paths will still be
guessed and appended to the base-urls. Additionally the servlet context
path is not read from the properties but from the ServletContext itself.

closes #488