DOC-1281: edits for Puppet Server external_ca_config
DO-1281: Puppet Server docs, external_SSL_termination

Edit and reorg of external_SSL_termination document for Puppet Server.
DOC-1281: edits to external_ca_configuration and external_SSL_termination pages

Edits and adding a note about X-Client-Cert.
Merge pull request #303 from jbondpdx/ca-doc

DOC-1281: edits for Puppet Server external_ca_config