Prevents spurious clientcert warnings in serverless mode

[binford2k]

When there are no clientcerts, Puppet will warn when it creates an
SSLContext for HTTPS operations. This situation occurs when you run
entirely serverless and never generate clientcerts. It's spurious in
that case, so we don't actually need to warn about it.

This behaviour was added in 3f7f830
so that the new HTTP client could download files via HTTPS from the
puppetserver (for example, the way that pe_repo) works.

To prevent this being a failure when running puppet apply in
serverless mode, it explicitly marks the clientcerts as optional in

puppet/lib/puppet/ssl/ssl_provider.rb

Line 98 in 06bc441

private_key = cert_provider.load_private_key(Puppet[:certname], required: false)

and

puppet/lib/puppet/ssl/ssl_provider.rb

Line 103 in 06bc441

client_cert = cert_provider.load_client_cert(Puppet[:certname], required: false)

This goes one step further and sets the output to INFO rather than WARN
when running puppet apply.

This does have one small edge case. If,

1. You intend to run a standard server/agent setup, and
2. Before ever running puppet agent -t you run puppet apply for
   provisioning purposes, and
3. Part of that Puppet run attempts to download a file from the
   puppetserver

Then you will get a certificate validation error and the HTTPS request
will fail silently with only an INFO message as a hint explaining why.

To fix it, you obviously just generate and sign the clientcerts.

I think this is an acceptable tradeoff, but would like other opinions.
This will need specs before merging.

Fixes #21