Remove build dependency on godep

Instead of using `godep path`, we can simply set the GOPATH directly to
point to the Godeps/_workspace. We can still use `godep` to manage the
dependencies on the Godeps/ tree, but we don't need to have it available
for straight builds from git.

v2: Rebased and moved to inside kube::setup_go_environment() function.

Tested:
- Built it without godep in $PATH:
  $ hack/build-go.sh
- Ran unit tests without godep in $PATH:
  $ hack/test-go.sh
- Retested after rebase.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

README.md

@@ -67,7 +67,7 @@ The commands above will not work if there are more than one directory in ``$GOPA
 
 ### godep and dependency management
 
-Kubernetes uses [godep](https://github.com/tools/godep) to manage dependencies. Please make sure that ``godep`` is installed and in your ``$PATH``.
+Kubernetes uses [godep](https://github.com/tools/godep) to manage dependencies. It is not required for building Kubernetes but it is required when managing dependencies under the Godeps/ tree. Please make sure that ``godep`` is installed and in your ``$PATH``.
 
 #### Installing godep
 There are many ways to build and host go binaries. Here is an easy way to get utilities like ```godep``` installed: