Add a new command to the CLI, get-gradle
This lists a subset of the known information about all gradle installations,
formatted as JSON.
On the jenkins server, this information is stored in the file
hudson.plugins.gradle.Gradle.xml.
What we are outputting here is a map, where the keys are the names of all of
the gradle installations, and the values are an array if id's for all of the
installers that correspond to the given installation.
The original source for this info is
GradleInstallation.DescriptorImpl.getInstallations().
This commit includes a bunch of commented out stuff as well that could serve
as a future reference for navigating through more of the data structure, if it
was desired for the output to be more comprehensive.  The next commit will
clean this up.
Cleanup only.  No functional changes.
Add a descriptive comment, remove commented out code.