Added test cases in ConfKeyManagerTestCase to verify fixed_key.

ConfKeyManagerTestCase sets the fixed_key option, but does not
perform checks to verify it was used.  This patch adds the
appropriate unit test cases to verify the fixed_key option.

Change-Id: I686b095eeb86b67271dc0e3cb5b6acd5a24d3d03
Closes-Bug: #1226228
Merge "Added test cases in ConfKeyManagerTestCase to verify fixed_key."