Stop using a private splinter API in the Cockpit test cases

[ncoghlan]

The Cockpit plugin test cases are currently using a private interface in the splinter API to check whether or not the Import App button has been enabled yet (import_button._element.is_enabled).

The splinter devs are open to the idea of providing a public API to check whether or a component is enabled, similar to the current one for checking component visibility: cobrateam/splinter#544

This issue tracks the steps needed to eliminate the use of the private API from the LeApp test suite:

• Implement the enabled checking API in splinter (RFE: allow checking for element enabled/disabled status cobrateam/splinter#544)
• Update the minimum required version of splinter to the first version including the new API
• Change the test suite over to using the public interface