Fixed gen_key to expect the proper PIV Key references.

0x9B is defined as the Card Management Key, and probably shouldn't be
regenerated.  0x9E is the Card Authentication key which is what you
should be generating keys for.  This also brings piv-tool in line with
the documentation that states 0x9A, 0x9C, 0x9D and 0x9E are the proper
keyIds to use.
Merge pull request #158 from cbancroft/fix-piv-tool-gen_key

Fixed gen_key to expect the proper PIV Key references.