mgmt_system -> psphere

Built on top of #1104. It seems I successfully fixed the issue.
Comparing this to the old one, it looks like I got rid of some useful functionality.

I'm thinking this is a closer approximation to the old behavior:
```python
def _get_resource_pool(self, resource_pool_name=None):
    if resource_pool_name is not None:
        return mobs.ResourcePool.get(self.api, name=resource_pool_name)
    else:
        return mobs.ResourcePool.all(self.api)[0]
```