[server/utils] update passlib API to 1.7 (closes #17054805)
while keeping bw-compat with 1.6 - 2.7 is not available for all distributions,
e.g. jessie.
The dependancy has been bumped in 6186cd15a46d because of a change in
bc9d901cb9e6. We may easily use the appropriate method depending on the version
installed.