improve speed of some ec2 keypair tests
improve speed of some ec2 keypair tests

test_create_key_pair_quota_limit is one of nova's slowest tests,
because it creates 11 cryptographic keypairs.

Similarly, test_create_key_pair is slow because it creates 7 keypairs.

Neither test actually cares that the keypairs are cryptographically
secure, so mock out the actual generation of the keypairs.

Change-Id: Id4034e9e50b04f17f68b555074fc5b4ffb3a82d7

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