plugin x509_certs fails when a key is included in local files

[benschweizer]

Software like haproxy require ssl keys to be in the same pem file like certs and chains.
Though, the plugins fails to handle that case when using local file input.

Relevant telegraf.conf:

[[inputs.x509_cert]]
  # parse a local pem file that includes cert+chain+key
  sources= ["/etc/ssl/private/something.pem"]
~

System info:

Telegraf 1.13.0 (git: HEAD 773e4ca)

Steps to reproduce:

1. create a pem file that contains a ssl key aside the certs
2. use local file as source

Expected behavior:

The plugin should just skip over the ssl key and only parse certificates.

Actual behavior:

The plugin aborts as the ssl key is not a valid certificate.

Additional info:

The pem module has an example how to parse this:

	block, rest := pem.Decode(pubPEMData)
	if block == nil || block.Type != "PUBLIC KEY" {
		log.Fatal("failed to decode PEM block containing public key")
	}