No reset password control

Found and fixed the following bugs on server:
1. When a PUT request is made by an active user to recover its password, there's non control that checks the validity of the new password. So:
- was created a new test, "test_put_active_user_weak_password" , which crashes and reveals this problem
- was added a password control into the "put" method of class Users 

2. Notice that function "pick_rand_pw" (used to generate a random complex password for tests) sometimes gives invalid passwords and tests crash.