Move go detection and environment setup into its own function

This way hack/config-go.sh can be used in environments where Go is not
available, such as running release/build-release.sh for Vagrant.

(The intention is to make config-go.sh a general library of helper
functions and import it from most other shell scripts.)

Fixes Issue #1057.

Tested:
- Built it and made sure it works.
  $ hack/build-go.sh
  $ output/go/bin/kubelet -version
  Kubernetes version 0.1+, build 0766e7a411c7-dirty

- Ran unit tests.
  $ hack/test-go.sh
  ok      github.com/GoogleCloudPlatform/kubernetes/examples      1.105s  coverage: 0.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/api       6.188s  coverage: 86.1% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/api/errors        1.015s  coverage: 81.8% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/apiserver 1.806s  coverage: 85.1% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/client    1.211s  coverage: 67.2% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/client/cache      1.115s  coverage: 95.5% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/cloudprovider/gce 1.052s  coverage: 7.3% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/cloudprovider/vagrant     1.045s  coverage: 76.8% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/constraint        1.038s  coverage: 100.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/controller        1.559s  coverage: 78.8% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/conversion        3.440s  coverage: 72.4% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/election  1.034s  coverage: 71.4% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/health    1.043s  coverage: 94.5% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/healthz   1.034s  coverage: 100.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/httplog   1.027s  coverage: 82.4% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/kubecfg   6.081s  coverage: 58.5% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/kubelet   1.400s  coverage: 72.2% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/kubelet/config    1.139s  coverage: 90.1% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/labels    1.041s  coverage: 98.7% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/master    1.033s  coverage: 33.3% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/proxy     1.095s  coverage: 86.5% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/proxy/config      1.038s  coverage: 39.2% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/binding  1.046s  coverage: 100.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/controller       1.039s  coverage: 43.6% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/endpoint 1.029s  coverage: 25.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/etcd     1.110s  coverage: 79.5% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/minion   1.048s  coverage: 72.3% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/pod      1.052s  coverage: 62.1% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/registry/service  1.054s  coverage: 80.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/scheduler 1.030s  coverage: 90.4% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/service   1.363s  coverage: 83.8% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/tools     1.136s  coverage: 81.9% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/util      1.049s  coverage: 83.9% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/util/config       1.036s  coverage: 92.9% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/util/wait 1.029s  coverage: 86.7% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/volume    1.032s  coverage: 83.3% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/pkg/watch     1.040s  coverage: 100.0% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/plugin/pkg/scheduler  1.026s  coverage: 90.9% of statements
  ok      github.com/GoogleCloudPlatform/kubernetes/plugin/pkg/scheduler/factory  1.075s  coverage: 85.4% of statements
  ?       github.com/GoogleCloudPlatform/kubernetes/test/integration      [no test files]

- Ran release/build-release.sh without go or godep in my $PATH.
  $ PATH=...
  $ which go
  $ which godep
  $ release/build-release.sh MYTEST
  Building release tree
  ~/devel/kubernetes ~/devel/kubernetes
  ~/devel/kubernetes
  Packaging release
  $ cat output/release/master-release/src/saltbase/pillar/common.sls
  instance_prefix: MYTEST-minion
  go_opt: -ldflags "-X
  github.com/GoogleCloudPlatform/kubernetes/pkg/version.gitCommit '0766e7a411c7-dirty'"
  $ find output/release/master-release/ | wc -l
  598

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