Raise Failure exception when setting duplicate other_config key
Raise Failure exception when setting duplicate other_config key

When https://review.openstack.org/7971 got merged (and fixed in
https://review.openstack.org/8504), it introduced a bug where a key
in the xenapi other_config would get set twice. The real xenapi would
raise a Failure exception in that case. So make sure the fake module
also does that to ensure similar bugs will get caught in the future.

Change-Id: Ie0151ffaf45dade930c3e8ff1279bfeab537bab2

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