Remove calls to self.cluster.call_async()

[jacobtomlinson]

Currently the implementations of VMInterface call self.cluster.call_async() when they want to run a synchronous function. This reference to the parent cluster instance means that VMInterface instances can't exist outside of a cluster.

This PR adds a VMInterface.call_async method which encapsulates the call to self.cluster.call_async(). This allows others to override this method if they wish more easily.

[armagankaratosun]

Hey again @jacobtomlinson 👋 ,

Sorry for commenting on a merged PR, but after trying out this change in the Openstack provider, I found out that VMInterface.call_async method recursively calls itself for the floating IP creation, eventually throwing an error;

maximum recursion depth exceeded in comparison

I quickly tried to use asyncio.to_thread instead;

    async def call_async(self, f, *args, **kwargs):
        """Run a blocking function in a thread as a coroutine."""
        return await asyncio.to_thread(f, *args, **kwargs)

and it seems to be working, but since I'm no expert on this topic, I wanted to know what you might think about this issue.

[jacobtomlinson]

Ah ok, can you open a new issue to track this?

[armagankaratosun]

Here; #460