Fix compatibility issue with passlib version < 1.6 (as encountered on eg EL6 systems)

- passlib.apache.HtpasswdFile's 'default_scheme' was called 'default'
before passlib version 1.6
- while here also deal with passlib.apache.HtpasswdFile's older
'autoload' paramter vs the newer 'new' one.
Merge pull request #4364 from stoned/pre-1.6-passlib

Fix compatibility issue with passlib version < 1.6 (as encountered on eg...