Add support for `copyArtifactToDisk` (true/false) param

[pvdissel]

If copyArtifactToDisk is true, the artifacts are downloaded to the local disk path as configured with the environment variable ARTIFACTORY_DATA_ARCHIVE, before being removed from Artifactory.

If false (default), artifacts are removed from Artifactory.

No existing functionality is changed, just refactored for readability, maintainability, and testability.

Includes:

• improved docs
• improved logging
• easier plugin development setup (incl. local Artifactory instance script, and gradle build)
• extensive tests of all non-time-based functionality (eg. not testing if an x many days old artifact is going to be removed)

[CLAassistant]

All committers have signed the CLA.