Update to GemFire 7.0.2

This brings Boot's dependency management into line with the version
of GemFire used by Spring Data GemFire in the Dijkstra release train
Rework HealthEndpoint and HealthIndicators

Refactored HealthEndpoint to be able to take multiple HealthIndicators. Extracted configuration of HealthIndicators out of EndpointAutoConfiguration and added new HealthIndicatorAutoConfiguration class.

Added HealthIndicators for Redis and Mongo.
Use the official JMS spec artifact
Remove spring-boot-starter-data-neo4j

The transitive dependency org.neo4j:neo4j-cypher-dsl:2.0.1 isn't
available in Maven Central. This was leading to frequent build breaks
so we've decided to remove the Neo4J starter until all of its
dependencies can be resolved from Maven Central.

Fixes #797
Remove some compiler warnings
Add constants to SecurityProperties and ManagementServerProperties

For the convenience of users who want to selectively override the
access rules in an application without taking complete control of the
security configuration we now have some constants:

* SecurityProperties.ACCESS_OVERRIDE_ORDER for overriding just the
application endpoint access rules

* ManagementServerProperties.ACCESS_OVERRIDE_ORDER for overriding the
application endpoint and management endpoint access rules

Fixes gh-803
Revert "Fix dependency management failure"

This reverts commit 1ef3d3bda9680b4c4517ef0a32a91250e15c2be8.
Remove spring-data release train BOM

Temporarily remove the spring-data release train BOM since it cannot
be resolved until it is published to maven central.

The previous solution of adding an additional repository has been
reverted as it will pollute POMS for users that inherit from the
spring-boot-starter-parent POM.
Fix broken documentation link
Allow meta-data driven version overrides in Gradle

Add a `versionManagement` gradle configuration which can be used to
provide alternative version meta-data. Primarily added so that the
Spring IO platform can provide version overrides without causing a
cyclic build dependency.

Fixes gh-750
Add unit test for (Mongo|Redis)HealthIndicator
Add @ConditionalOnProperty annotation

Add @ConditionalOnProperty which enables conditional bean creation when
all required properties are defined in the Environment.

Fixes gh-812
Merge pull request #812 from maciejwalkowiak/conditionalOnPropertiesPresent

* conditionalOnPropertiesPresent:
  Add @ConditionalOnProperty annotation
Rename SimpleHealthIndicator to SimpleDataSourceHealthIndicator
Rename SimpleHealthIndicator to SimpleDataSourceHealthIndicator
Update documentation for new HealthIndicator support
Rename missing test
Merge pull request #819 from cdupuis/health-indicator

Rework HealthIndicator support
Add missing tests
Add gc information to /metrics endpoint