fixes #5182 - Rename Systems to Content Hosts

This commit contains minor changes to rename Systems to Content Hosts
(i.e. system -> content-host).  Note: although the command name is
changing, the CLI will continue to use the 'systems' API.

This commit does not address renaming System Groups as that will be
handled separately.
Merge pull request #145 from bbuckingham/issue-5182

fixes #5182 - Rename Systems to Content Hosts