Add variable interpolation for the distribution picked into the jvm platform argumetns

[ericzundel]

We are currently using pants 0.0.53

Here's what we currently do to specify bootclasspath:

[jvm-platform]
default_platform: 1.8
platforms: {
    '1.7': {'source': '1.7', 'target': '1.7', 'args': ["-C-Xbootclasspath:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/sunrsasign.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre/classes", "-C-Xlint:cast", "-C-Xlint:deprecation", "-C-Xlint:empty", "-C-Xlint:finally"] },
    '1.8': {'source': '1.8', 'target': '1.8', 'args': ["-C-Xbootclasspath:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/sunrsasign.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/classes", "-C-Xlint:cast", "-C-Xlint:deprecation", "-C-Xlint:empty", "-C-Xlint:finally"] },
  }

[jvm-distributions]
paths: {
    "mac os x": ["/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home", "/Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home"],
    "linux": ["/usr/java/jdk1.8.0_45", "/usr/java/jdk1.7.0_80"],
  }

In practice, we generate this file. Currently , it generates linux paths for -Xbootclasspath: on Linux and mac paths when you exec pants on a mac. What would be better is the ability to do some kind of string interpolation inside one of the 'args' variables for the platforms setting.

Something like:

[jvm-platform]
default_platform: 1.8
platforms: {
    '1.7': {'source': '1.7', 'target': '1.7', 'args': ["-C-Xbootclasspath:(%distribution)/jre/lib/resources.jar:(%distribution)/jre/lib/rt.jar:(%distribution)/jre/lib/sunrsasign.jar:(%distribution)/jre/lib/jsse.jar:(%distribution)/jre/lib/jce.jar:(%distribution)/jre/lib/charsets.jar:(%distribution)/jre/lib/jfr.jar:(%distribution)/jre/classes", "-C-Xlint:cast", "-C-Xlint:deprecation", "-C-Xlint:empty", "-C-Xlint:finally"] },
...