Ldap Authentication: Improve UI by disabling password field at /admin/people/create when using only LDAP login

When synching only from LDAP to Drupal, not importing the passwords from LDAP, and allowing only LDAP login, I was confused about what password Drupal stored in the Drupal 'users' table (closed this: https://drupal.org/node/2030867#comment-7607875). It turns out that a 20 character random password is generated for the account on line 1213 of LdapUserConf.class.php. This patch improves the UI on this topic by providing an option on /admin/config/people/ldap/user to disable the password field at /admin/people/create and by explicitly mentioning that a random password will be generated in this scenario. It also logs a watchdog entry to confirm that a random password was generated when a new user account is created.