Support packaging hst repository configuration with either repository or site application

The Robots.txt, Sitemap.xml and Poll plugins all provide hst repository configuration (hcm-module).

With the new Multi-Site feature such hst configuration by default now must be bundled and packaged with the site web application.

Both the Robots.txt and Poll plugin also provide 'global' repository/bootstrap configuration (like for example the robotstxt.cnd and poll.cnd) which must not be packaged with the site web application.

The Sitemap.xml plugin only provides hst repository configuration.

For all three plugins a new module (jar artifact) will need to be created which separates out the hst specific repository configuration.

Note though: because we still intend to support the old project structure (only feasible when using a single site application) we cannot simply 'move' or merge the hst repository configuration for the Robots.txt or Poll plugin with their existing hst client module!
This really requires introducing new/separate -hcm-hst jar modules.

One these are available they will need to be added to the release pom (dependency management) and accounted for in Essentials installation instructions.

Furthermore, upgrade documentation will be needed for existing projects using these plugins to add this new dependency to the appropriate module (cms, site/webapp).