Optional heap and permgen args for java VM

Allow optional `$jenkins_max_heap_size` and `$jenkins_max_perm_size` vars to control heap and permgen sizes in Jenkins Java VM via `java` command line args.

Usage:

    $jenkins_max_heap_size = "6G"
    $jenkins_max_perm_size = "512M"
    include jenkins

Both variables are optional. Arguments to `java` are only added if the corresponding variable is set.