Updating CONTRIBUTING doc to discourage "setup.py develop".
Updating CONTRIBUTING doc to discourage "setup.py develop".

In the process, also

- Replacing env. var. refs. "$FOO" with "${FOO}"
- Replacing references to "~" with "${HOME}"
- Removing outdated workarounds using ``gcloud`` CLI

dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python,jgeewax/gcloud-python,dhermes/google-cloud-python,tseaver/gcloud-python,daspecster/google-cloud-python,dhermes/google-cloud-python,daspecster/google-cloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,quom/google-cloud-python,jonparrott/google-cloud-python,calpeyser/google-cloud-python,tswast/google-cloud-python,jonparrott/gcloud-python,Fkawala/gcloud-python,quom/google-cloud-python,tseaver/gcloud-python,dhermes/gcloud-python,tartavull/google-cloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,Fkawala/gcloud-python,tartavull/google-cloud-python,calpeyser/google-cloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,jgeewax/gcloud-python,GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python