Add system test for Database convenience methods:
Add system test for Database convenience methods:

- batch
- snapshot
- read
- run_with_transaction
- execute_sql

Closes #3015.

tseaver/google-cloud-python,tswast/google-cloud-python,calpeyser/google-cloud-python,tseaver/gcloud-python,tswast/google-cloud-python,tartavull/google-cloud-python,tseaver/google-cloud-python,tartavull/google-cloud-python,googleapis/google-cloud-python,dhermes/gcloud-python,dhermes/gcloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,GoogleCloudPlatform/gcloud-python,calpeyser/google-cloud-python,dhermes/google-cloud-python,GoogleCloudPlatform/gcloud-python,jonparrott/gcloud-python,tswast/google-cloud-python,tseaver/gcloud-python,tseaver/google-cloud-python