Password management

[leogr]

TODOs:

• [ ] check security consideration: https://tools.ietf.org/html/rfc7643#section-9.2
• [ ] ensure that the value is encrypted ( depends on #48 )
• [ ] define service provider policy (ie. who can change the password?)
• [ ] understand the meaning of

  "changePassword": {
    "supported": false
  },

  within the Service Provider configuration

• [ ] evaluate https://tools.ietf.org/id/draft-hunt-scim-password-mgmt-00.txt