Clarify ipaserver_external_cert_files variable type

[myllynen]

In ipaserver/README.md example ipaserver_external_cert_files has a string value assigned but the description later in the page states it should be a list. It would be nice to fix this inconsistency, based on the variable name the description sounds more likely being correct. Thanks.

[t-woerner]

It is indeed a list, but it is possible to use single string that is used like ["single string"].

[myllynen]

Ok in case of it being a list adding those brackets would make it clear that that it's a list for those reading only the example.

So instead of the current form

ipaserver_external_cert_files: "/root/chain.crt"

it should be read

ipaserver_external_cert_files: ["/root/chain.crt"]