Prepare to externalise JRE configuration

Delete JRE vendor Oracle. Oracle is not a supported JRE vendor since
there is no stable download location for Oracle JREs and Pivotal
cannot host these JREs.

Delete JRE type attribute. Since we control all the JREs that can be
downloaded by this buildpack, require them to be in tar.gz format.

Delete JRE id attribute. Reduce the surface area of the JRE selection
support prior to replacing the implementation. Move the format printed
by the detect script into Detect where it can evolve more
appropriately without impacting the JRE selection logic.

Split SelectedJre into two classes

* JreProperties encapsulates system.properties file and environment
variable handling for JRE vendor and version.

* JreSelector encapsulates mapping JRE vendor and version to a URI
from which to download a tar.gz file.

[#50171283]
Merge 50171283-externalise-jre-configuration to master

[Completes #50171283]