Adding ChefSpec::Coverage to spec_helper.rb

Change-Id: I1adc0f186f2b8ae33fbc5e4b5a2e5f7ed60ed2e5
Addresses: blueprint lint-and-unit-testing-for-havana
Utilise ['openstack']['region'] from cookbook-openstack-common

- Set ['openstack']['metering']['region'] to equal ['openstack']['region']
- Add test to override the endpoint and confirm it is used.

Change-Id: Ie43ed28301ab40902f43b67831f5c5ff5300c65f
Addresses: blueprint uniform-region