Don't rely on GOOGLE_CLOUD_PROJECT
Don't rely on GOOGLE_CLOUD_PROJECT

Because SERVICE_ACCOUNT is built using a project identifier from the
Pub/Sub client, SERVICE_ACCOUNT was changed to an instance variable
so it could be set after the Pub/Sub client is instantiated.

Changed all other constants to instance variables for consistency and
simplicity.

GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples