fix(convert_pem_str_to_cert): always return the first cert

[jimethn]

All usages of it expect it to return a single cert, so if the files contain multiple certs it crashes the program anyway.

[Kishi85]

The reason why convert_pem_str_to_cert can return a list is to allow for intermediate CAs to be provided (not a use-case when using Let's Encrypt) when parsing the CA part of the ACME response and this PR would break that behaviour by just returning the first CA of the chain.

Could you please open an issue for this providing a backtrace of the crash so we can fix the broken part that does not work with a list return value (at first glance the issue might be tool.is_cert_valid). Also likely depends on which ACME provider you are using as this does not happen with Let's Encrypt otherwise it would have occurred during testing.