Fix TLS documentation.
Support TLS certificate chain files.

This aligns Munin TLS setup with other TLS clients and servers:
 * tls_certificate is used for specifying the client or server
   certificate and optionally intermediate CA certificates (if any)
 * tls_ca_certificate is used only for specifying CA certificates used
   to verifying peer's certificate (as documented) and not for
   specifying intermediate CA certificates

This allows separation of intermediate client and server CA certificates.
Merge pull request #313 from eehakkin/feature/certificate-chain-file

feature/certificate chain file