[google|sql] Initial support for Google Cloud SQL

This commit adds the initial support for Google Cloud SQL:
- Adds a new service 'SQL' to the existing 'Google' provider
- Creates new shared methods to be reused by different services
- Add requests, models and tests for Tiers
[google|compute] added centos, opensuse images
Merge pull request #3065 from erjohnso/gce_moar_images

[google|compute] added centos, opensuse images
Merge pull request #2977 from frodenas/gce_sql

[google|sql] Initial support for Google Cloud SQL
Added backend services resource to gce compute
Added backend services example and model classes
Merge pull request #3069 from snyquist2/gce_l7

[google|compute] Add backend services resource for L7 load balancing
Whitespace cleanup of #3069.