Allow overriding of Kudu github URL
Allow overriding of Kudu github URL

This is a convenience to allow developers to build a branch of
kudu from a different github account (e.g. their own fork).

Testing:
Tested it locally on a private github enterprise repo. The kudu
build is still in progress but it successfully downloaded the
tarball and started building it.

Also confirmed that it still works if the URL is unspecified.

Change-Id: I181c599816435ca4997d84a6cd90386c3841ce42

timarmstrong/native-toolchain,cloudera/native-toolchain,mjacobs/native-toolchain,cloudera/native-toolchain,cloudera/native-toolchain