Profitbricks

* Save the credentials to the keyring with an unique identifier.
* Minor fix to a user management test.
* Fix for the issue when create LAN/LB/IPBlock replaced the entity name because of it contains _ character in it. Related to [this issue](https://github.com/profitbricks/profitbricks-module-ansible/issues/20).

* Adjusted authentication logic to use `requests` library auth; this corrects expected auth order.
* Updated documentation.

Extended depth on the locations so that `imageAlias` can be retrieved for a required location.

* 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.