Add a directory for core_modules, the main reason to use a separate
directory over vendor_modules is also so they are not on the modulepath
of the current puppet masters.  Once everything is running puppet7 we
should merge this Puppet file and the core_modules directory with the
default Puppetfile and the vendor_modules directory

Also add the following modules
 * nagios_core
 * sshkeys_core

Bug: T330490
Change-Id: Id509606fb905e554dbe763aadef959672bc40ec1