Use files downloaded during internet detection

This avoids the first file downloaded to determine internet
availability from being downloaded again unnecessarily.

[#57792554]
Merge 57792554-optimise_initial_download to master

[Completes #57792554]
Add Spring Insight documentation

[#59671032]
Merge 59671032- to master

[Completes #59671032]
Add Tomcat datasource JAR, if present, to WEB-INF/lib

[#59314610]
Merge 59314610-tomcat-datasource to master

[Completes #59314610]
Fix rubocop issues

[#59314610]
Merge 59314610-tomcat-datasource to master

[#59314610]
Support for PropertiesLauncher

Spring Boot has a third kind of launcher, the PropertiesLauncher.
Previously, we did not add a --server.port flag when starting this
kind of launcher.  This change ensures that we do.

[#58809612]
Merge 58809612-properties-launcher to master

[Completes #58809612]
Link to system tests
Documentation fix

Previously the ssl-related key name for the AppDynamics framework was
documented incorrectly.  This change fixes is.

[#52415605]
Merge 52415605-doc-bug to master

[Completes #52415605]
[#59817096]Support Errno::ENETUNREACH

The internet availability check was failing because Errno::ENETUNREACH
was not being handled.

This fix adds Errno::ENETUNREACH and a number of other network-related
exceptions to the set being handled. When these errors are caught on
the first download attempt in a buildpack invocation (detect, compile,
or release), the internet is deemed to be inaccessible and a look-
aside is performed in the (currently read-only, but pre-populated)
buildpack cache.

See https://www.pivotaltracker.com/s/projects/803319/stories/58941750
for background.

[#59817096]
Merge 59817096-ENETUNREACH to master

[Completes #59817096]
Merge branch 'master' of github.com:rmorgan/java-buildpack