Simplify compute-service

This aims to remove some of the confusion around the compute-service functions.

This makes p.api/compute-service forward to p.configure/compute-service.

p.configure/compute-service no longer tries to instantiate a provider, and is
limited to looking up service configurations.

p.compute/compute-service is renamed p.compute/instantiate-provider.
Format remote-directory and remote-file docstrings
Updated project.clj, release notes and readme for 0.8.0-beta.2
Merge branch 'release/0.8.0-beta.2'
Merge tag 'pallet-0.8.0-beta.2' into develop

pallet-0.8.0-beta.2
Updated version for next release cycle