Fixed a problem of changing mysql passwords

There was a chicken and egg problem discussed in issue #5918:
"Setting mysql root password the first time"

Now the following SLS file works for me:
```YAML
set_localhost_root_password:
    mysql_user.present:
        - name: root
        - host: localhost
        - password: {{ pillar['mysql.pass'] }}
        - connection_pass: ""
        - watch:
            - pkg: mysql-server
            - service: mysqld
```
It sets mysql root password the first time and does not fail after that.