Createrepo password improvement

In order to create more secure passwords for libraries, the **minimum password length** is set at 8 characters.

Also a **random password generator** is present. This generates 256 random bits and encodes them to base64. The resulting string is the random password.

Finally, a **password strength checker** is used. It uses entropy calculation of the password string. The minimum accepted entropy level is set to 18. Below this entropy the password is not accepted. Medium and Strong levels are set to 22 and 25 entropy bits accordingly.