Add Google Cloud Storage support

[lnovara]

The aim of this PR is to introduce support for Google Cloud Storage (GCS) as a backend for furyagent.

Summary of changes:

• added support for GCS
• removed commented code for Swift (I don't think we'll ever need it)
• fixed permissions of the directories created by furyagent

Since it is not possibile to mock the Google API, I've tested the changes by creating the needed resources using Terraform and correctly configured furyagent to use them.

[lnovara]

Since this PR was merged, I am tagging the 0.0.7 release.