Support 20 byte passwords
Support 20 byte passwords

The IPMI 2.0 spec allows 20 byte passwords.
If a password is less than 16 bytes, then use
the 1.5 scheme as before.  If between 16 and 21,
then attempt IPMI 2.0 scheme.  Raise when over 20.

Change-Id: I25016b8608d9dbf5d436afd75bd07f0d4eed6d81

openstack/pyghmi,openstack/pyghmi,benoit-canet/pyghmi,stackforge/pyghmi