Configurable default for custom libraries and descriptors

[cliffano]

Custom Image Provisioner, Custom Stack Provisioner, and descriptor URLs are currently configurable at pipeline runs.

Need to add configuration properties for them so users can fill in default values.
This will then help future pipeline runs where the person triggering the build no longer needs to provide URLs to those artifacts, and instead, the person only needs to modify the default - usually this involves changing a version number.

[cliffano]

Done. To be included in initial release.