Sanitize JDKs on the AIX machines

[sxa]

We currently have a mix of JDKs on the AIX machines - and some default to a J9 VM instead of Temurin (Either 7 or 8), some have no default, and this can lead to inconsistent behavior.

The build machines generally seem to have a Temurin 8 available and defined by the JDK7_BOOT_DIR variable in the machines' jenkins definitions. We should ensure that this is consistent across all of our AIX machines. We could also use this opportunity to start using "well-known" locations for the boot JDKs - specifically locations that are searched by default by the openjdk build process that would not require the variables to be set.

Identified in #2288 (comment) where an ineffective keytool was being picked up when scheduling a JDK8 build on a test box.