Support default project for Pubsub

[jabubake]

All the veneer libraries support picking up the default project id if an explicit one is not provided.
See Spanner , Datastore, Storage :
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

The deprecreated PubSub api supported it as well, v0.9.2-alpha does not.

[lesv]

I know this doesn't block GA, but it should block BETA - I don't see such a tag.

[garrettjonesgoogle]

We are using "Status: Release Blocking", and the API it applies to disambiguates between GA blocking and beta blocking.

[garrettjonesgoogle]

Renamed title - this is for default project, not default auth (which is already supported).

[garrettjonesgoogle]

As a workaround for Beta, could we have the samples use ServiceOptions.getDefaultProjectId()? This would just be one extra line.

https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-core/src/main/java/com/google/cloud/ServiceOptions.java#L339