WIP this doesn't work yet
Because the same service may exist in multiple providers, e.g.
IdentityV2 will be in both Rackspace and HP, it became necessary
for the ServiceDiscovery to register the provider's name when
registering the "base_provider_path".

What also shook out of this is that "base_provider" probably
only needs to be stated once: when the provider is registered
with ServiceDiscovery.

Now, providers register with a single call as follows:

```ruby
ServiceDiscovery.register_provider(
	'openstackcore',
	'Fog::OpenStackCore',
	'the/path/to/osc/services'
)
```

and ServiceDiscovery is now instantiated as follows:

```ruby
ServiceDiscovery.new('openstackcore', 'identity', options_hash)
```
Merge pull request #26 from fog/base_provider_path

[ServiceDiscovery] Now with Fewer Args At #new Time™