Ionosenterprise

* Added config file and keyring support
* Added profitbricks.utils containing helper functions: ask() and find_item_by_name
* Added ProfitBricksService.get_datacenter_by_name()
* Implemented ProfitBricksService.wait_for_completion()
* Added support for providing a client user agent
* Added pb-api-shell script to examples
* Flake8 and pylint style fixes - refactoring
* Fixed adding user to group & some flaky test

The SDK has been updated to support the latest ProfitBricks Cloud API v4 features:

* Added User Management feature
* Added Contract Resources feature
* Added Image Aliases feature
* Added a new location and license type
* Added IP Address fail-over feature
* Removed commented code
* Added and updated tests
* Added additional negative tests

Tests and documentation have been improved.

* Improved documentation.
* Improved examples in the README.
* Improved test coverage.

Update includes the following:
- PB exception for rate limit exceeded.
- Removed broken download URL from setup.py.

This release includes the following changes.
- Added custom ProfitBricks errors as described in 6.
- Fix passing of mutable values as default arguments.
- Cleaned up example scripts.
- Apply flake8.

This updates the ProfitBricks SDK for Python to the ProfitBricks Cloud API v3. Changes include the following:
- Updated Cloud API REST URL.
- Added support for volume availability zones.
- Added support for NAT.
- Set `User-Agent` header with SDK name and version.
- Changed `Content-Type` to `application/json`.