Improve compiler error when providing a wrong attribute

[bartv]

When a wrong attribute is used the following error is given:

Error: no attribute gpg_key on type yum::Repository (reported in gpg_key (model.cf68:9))

We can improve this by:

• Adding the list of attributes that are available to the message
• If there is a highly similar one suggest that

For example:

Error: no attribute gpg_key on type yum::Repository (reported in gpg_key (model.cf68:9)). Did you mean `gpgkey`? The following attributes are available name, host, gpgkey, baseurl, ...