Improve @LocatorApplication to configure the Apache Geode locators property [DATAGEODE-319]

[spring-projects-issues]

John Blum opened DATAGEODE-319 and commented

As a Spring application developer building Apache Geode Locator applications configured and bootstrapped with Spring (Boot), I'd like an easy way to configure my Spring (Boot), Apache Geode Locator application to join an existing Geode cluster, perhaps configured/started using Gfsh, as a member.

Technically, this means the @LocatorApplication annotation (Javadoc), should maintain a locators attribute and additionally process the spring.data.gemfire.locators property in the same way that a @PeerCacheApplication annotated Spring Boot application can, as seen here.

This entails:

1. Add in a new locators attribute to the @LocatorsApplication annotation.
2. And modify the configuration class to configure the Apache Geode locators property from the spring.data.gemfire.locators property, falling back to the locators annotation attribute

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Backported to: 2.2.7 (Moore SR7)