Automatic creation of users with ldap authentication

[c8y3]

In relation to #203, this pull-request adds the creation of users present in ldap at their first login.
The behaviour is activated by setting variable IRIS_AUTHENTICATION_CREATE_USER_IF_NOT_EXIST to True.
The following variables were added:

• LDAP_SEARCH_DN is the base distinguished name to perform the user search (like ou=users,dc=example,dc=org)
• LDAP_ATTRIBUTE_IDENTIFIER is the name of the attribute used to build the search filter which will uniquely identify the user (like cn or sAMAccountName)
• LDAP_ATTRIBUTE_DISPLAY_NAME is the name of the attribute to retrieve the user display name (the user login will be used if this variable is not set)
• LDAP_ATTRIBUTE_MAIL is the name of the attribute to retrieve the user email (the fake email user_login@ldap will be used if this variable is not set)

A fake local password is chosen randomly (same way as the administrator password is set). Ideally, it would be better if the database accepted the creation of users without any local password. This could be a further improvement of the code.
Note that the email unicity requirement may prove to be a problem at some point (if two users happen to have the same email in ldap, this may be possible in theory)
Note that in case of NTLM, the domain name is stripped to get the value of the sAMAccountName.