Fix some issues in wordpress-nginx

I run `wordpress-nginx` playbook in Google Compute Engine. 
Because GCE don't allow ssh as root, I use sudo.

I found a few small errors when running it then I fix it.

```
 ____________________________________________________
< TASK: common | Copy the EPEL repository definition >
 ----------------------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||


failed: [xxx.xxx.xx.xx] => {"failed": true, "item": ""}
msg: Aborting, target uses selinux but python bindings (libselinux-python) aren't installed!

FATAL: all hosts have already failed -- aborting
```

```
 ___________________________________________________________
< TASK: wordpress | Fetch random salts for Wordpress config >
 -----------------------------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||


failed: [xxx.xxx.xx.xx] => {"failed": true, "item": "", "parsed": false}
invalid output was: [sudo via ansible, key=ilbqinrqrcryuizfbwuygcqardkmqjft] password: 


FATAL: all hosts have already failed -- aborting
 ____________
```