AS-288: Check deleted CLB when adding node (review)

This will reduce unwanted retries which will in-turn succeed invalid load balancer tests. I have factored out the checking of deleted CLB in separate function and used it in add and remove node code.
IIUC, this is a TestCase without tests; it's intended only as a superclass. That works, mostly: trial misdiagnoses is as a test case, but as long as no test methods are added to this class that are intended to be parametrized by subclasses.

So, two suggestions, both optional:

1. Composition rather than inheritance
2. Make this a mixin that subclasses from object, rather than a test case itself, and only add the test case superclass to classes that actually contain tests.

I do the latter a lot but only because I've written a ton of parametrized tests :)