Attributes were hardcoded in some part of the php script

Attributes for username and mail where hardcoded in some part of the code. As it is configured on the wordpress setting, I replaced the hardcoded attributes references by the one specified on the setting
I did a change important here to notice :

```['uid'][0]  => [OPENAM_WORDPRESS_ATTRIBUTES_USERNAME]```

Notice that we used to select the first value of an array. In my environment, the value returned by OpenAM wasn't an array but directly the value.

A good things to do is to check in a different environment that this change doesn't broke everything