PubSub: Add support for emulator host variables

[danieletorelli]

Works whether the PUBSUB_EMULATOR_HOST variable is defined as a property or as environment variable.

Refs: #286

[lightbend-cla-validator]

Hi @danieletorelli,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

http://www.lightbend.com/contribute/cla

[danieletorelli]

CLA signed.

[johanandren]

Can we also use it for the tests?

[danieletorelli]

Would be great but, if I remember well, the emulator would require an authenticated Google Cloud SDK on the workstation to work correctly. The purpose of this PR is to allow developers that already have valid GCloud credentials and active Pub/Sub services to emulate the production environment on the local machines.

[johanandren]

Ah, thanks for clarifying.

[patriknw]

LGTM

[stephennancekivell]

hey, nice addition @danieletorelli ! Its cool to see someone else is using this.