Introduce GcscliBlobstoreClient Read-Only functional tests
Introduce GcscliBlobstoreClient Read-Only functional tests

These are the previously mentioned tests which were blocked on
bosh-gcscli implementing support for 'credentials_source' of
value 'none'.

These are significant tests as the majority of runtime
functionality is typically going to be as a client accessing
read-only buckets.

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh