Added default identity to Identity, Computer and Base providers.

### Changes
- Added the CloudIdentity object as an optional parameter in the constructor of the providers (Identity, Compute and ProviderBase).  This is used as the default identity when one is not passed into the provider methods.  
- Modified the tests to use the default provider instead of always passing them into the methods.
- Added ability to get the internal endpoint url from the service catalog (assuming one exists for the provider you are using).