The problem was introduced in commit f5789e8e. 'tenancy' is a parameter of
ec2.run_instances, but not in ec2.request_spot_instances. So it was breaking
the support for spot requests.
As stated in #423, the commit 7f11c3d broke ec2 spot instance launching
after 1.7.2. This is because it acts on the 'res' variable which have 2
different types in the method, and in case we request spot instances,
the resulting object is not a result of ec2.run_instances() but
ec2.request_spot_instances(). Actually this fix doesn't seem to be
relevant in the spot instances case, because by construction we won't
retrieve 'terminated' instances in the end.
