Previously we did a validation pass over CA PEM files before calling
Go's CertPool.AppendCertsFromPEM to provide more detailed error messages
than the stdlib provides.

Unfortunately our validation was overly strict and rejected valid CA
files. This is actually the reason the stdlib PEM parser doesn't return
meaningful errors: PEM files are extremely permissive and it's difficult
to tell the difference between invalid data and valid metadata.

This PR removes our custom validation as it would reject valid data and
the extra error messages were not useful in diagnosing the error
encountered.

Fixes an issue where the Docker API client would timeout before the kill
timeout was hit.

https://github.com/hashicorp/nomad/pull/4441

This reverts commit c17e0fc9.

This reverts commit 84926f75.