Allow install of google_auth_proxy from alternative sources

This patch set will allow the user to specify the node[:google_auth_proxy][:source] attribute in order to change the installation source of the google_auth_proxy binary.

* If a github uri compatabile with the golang_package LWRP is provided then the cookbook will attempt to use the golang_package LWRP to install the google_auth_proxy binary via "go get"
* If a uri starting with http:, https: or file: is provided, the cookbook will attempt to install the google_auth_proxy binary via the remote_file provider
* If the attribute node[:google_auth_proxy][:checksum] is specified then the remote_file provider will use it as the remote_file checksum

I've also added test-kitchen scripts & fixtures to allow for easier testing of the cookbook