Allow specifying custom Gradle version/distro/install

[eskatos]

For testing purposes it would be useful to be able to use a custom Gradle version, distribution or installation.

The Gradle version or path to distro/install could be set as a test annotation or in the HOCON configuration file leveraging environment variable substitution or including a generated HOCON file.

[eskatos]

It turns out that it's already possible to set a custom Gradle installation using system properties.

exemplar/sample-check/src/main/java/org/gradle/samples/test/runner/GradleSamplesRunner.java

Line 59 in d0ee8f7

final String gradleHomeDirProperty = System.getProperty("integTest.gradleHomeDir");

Doing that on a Test task makes sense.

It would be good for this library to include a sample (inception!) to demonstrate how to do it.

[lacasseio]

gradleConnector.useInstallation is ignored when samplesRootDir is configured.

[eriwen]

This is documented in 6e34c61 and #9 has been merged.