[IMP] auth_crypt: port to passlib instead of using embedded/hand-rolled md5crypt
[IMP] auth_crypt: upgrade default kdf to pbkdf2_sha512

Applies to new and yet-to-be-encrypted users, existing already-encrypted will
keep logging in as usual, their password storage will be upgraded on next
password *change*.
[IMP] auth_crypt: automatically upgrde deprecated hashes on login
[IMP] auth_crypt: simplify get_pw
[IMP] auth_crypt: variables naming
[IMP] auth_crypt: port encrypt-at-install from f29ff5e

also move utility methods to class bottom to improve reading experience
(hopefully)
Merge pull request #640 from xmo-odoo/master-auth_crypt_passlib-xmo

[MERGE] auth_crypt: Upgrade to pbkdf2_sha512

replace handrolled KDF by passlib (& usage of passlib API)
* replace handrolled KDF by passlib (& usage of passlib API)
* replace md5crypt by pbkdf2(sha512)
  - handle upgrade from an old database (rehash on login)
* forward-port encrypt-at-install from f29ff5e