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>

docs/getting-started-guides/gce.md

@@ -9,7 +9,7 @@
 4. You must have Go (version 1.2 or later) installed: [www.golang.org](http://www.golang.org).
 5. You must have the [`gcloud` components](https://developers.google.com/cloud/sdk/) installed.
 6. Ensure that your `gcloud` components are up-to-date by running `gcloud components update`.
-7. Install godep. [Instructions here](https://github.com/GoogleCloudPlatform/kubernetes#installing-godep)
+7. Install godep (optional, only required when modifying package dependencies). [Instructions here](https://github.com/GoogleCloudPlatform/kubernetes#installing-godep)
 8. Get the Kubernetes source:
 
         git clone https://github.com/GoogleCloudPlatform/kubernetes.git

docs/getting-started-guides/vsphere.md

@@ -18,7 +18,7 @@
    go get github.com/vmware/govmomi/govc
    ```
 
-5. Install godep. [Instructions here](https://github.com/GoogleCloudPlatform/kubernetes#installing-godep)
+5. Install godep (optional, only required when modifying package dependencies). [Instructions here](https://github.com/GoogleCloudPlatform/kubernetes#installing-godep)
 
 6. Get the Kubernetes source: