minor refactor in platform driver's get_resource_capabilities

with new option to retrieve actual capability enum class attributes (instead of the corresp values, which continues to be the default behaviour). This will help make the mission executive more general.

@edwardhunter please merge

@bobfrat :  You can then call:

```python
    res_cmds, res_params = self.platform_agent._plat_driver.get_resource_capabilities(cmd_attrs=True)
````
and `res_cmds` will be the actual class attributes, so this will help with your scheme of things in the mission executive.