(PDB-107) Support chained CA certificates

This patch makes puppetdb load all of the certificates in the CA .pem
file into the in-memory truststore. This allows users to use a
certificate chain (typically represented as a sequence of certs in a
single file) for trust.
Merge pull request #788 from grimradical/PDB-107-cert-chains

(PDB-107) Support chained CA certificates