Fix AESHSM to work with Python3
Fix AESHSM to work with Python3

The bytes() return for empty data must not contain an empty string
otherwise we will get a TypeError.

Also added the possibility to set the slot to -1, since with
AWS CloudHSM the slot number is unknown.

privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea