Fix KEK generation in the P11 Plugin + fix unit tests

Buckle up, this is going to be a long commit msg.

Background:
* Henry Yamauchi noted that the bind_kek_metadata method was returning
  an object early. This return prevented the entirety of the code that
  created the KEK on the HSM from executing
* This was not caught because the mock tests that checked for invocation
  of the method were wrong. Specifically they called methods like
  assert_called_once() and assert_called_twice(). Unfortunately, while
  assert_called_once_with(), et al are real methods on the mock objects
  that really do assert things, assert_called_once() is not. And when
  you call a method that doesn't exist on a mock object it returns
  another mock object. So many of these tests were testing nothing.
Fixes:
* bind_kek_metadata has been refactored to separate out the kek
  generation into a new method (_generate_kek) to make the expected flow
  clearer. And, of course, the bug has been fixed
* The tests have had all the incorrect assert calls removed. In some
  cases the call was actually unnecessary (typically because the test
  already used a return value from the mock so testing that it was
  called was not needed). In other cases an assert was added that
  checks that the called method on the mock object is true

Change-Id: I17a7adf7e3f386f3b0474e79731e7faab8911481