Core: Fixed latest 'excon' which breaks the VCR fixtures

The 'excon' was updated it 0.17.0 which ultimately breaks
the VCR fixtures we are using (rake test:drivers:google).

Saying that we don't want to update it in gemspec should
fix this issue.
Cimi frontend initializers
Tests: Removed debugging 'puts' from openstack tests
Core: Added possibility to define multiple features for collection

Instead of:

feature :collection, :name
feature :collection, :second_name

You can now use:

feature :collection, :name, :second_name, ...
Mock: Require 'yaml' in client instead of driver

The YAML is really required in MockClient (I think this
require is not required here anyway ;-)

This patch will also make .store() method return the stored
object, so in driver we don't need to create a special var for it.
Mock: Cleanup in how we create mock 'metrics'

Moved the code to the Metric model to keep MockDriver
clean.
Tests: Be more verbose when checking state in Mock tests

The XML should reflect the state change of an Instance.
Instead of checking the status code returned we should also
verify the XML.