Plugin repository used by init scripts can be configured

[erichaagdev]

There may be cases where a user can not resolve artifacts from the Gradle Plugin Portal and must instead rely on an internal artifact repository.

Currently, the initialization scripts are hard coded to use the Gradle Plugin Portal. We should consider making this configurable (a good candidate for #287).

Note: This is already possible to do in the TeamCity Build Scan plugin: etiennestuder/teamcity-build-scan-plugin#78