Only load host keys when reject_unknown_keys is True.
Only load host keys when reject_unknown_keys is True.

This allows users to sidestep Paramiko's hardcoded behavior of raising an
exception when host keys do not match (which is a distinct condition from not
*having* a key for a given hostname or IP address).

Signed-off-by: Jeff Forcier <f3e731dfa293c7a83119d8aacfa41b5d2d780be9@bitprophet.org>

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