[os][rpm] Support CentOS via its own class, stub an OEL
class, small fixes to redhat.pp to be generic enough for use
by these RHEL-ish operating systems, edits to allow EL7-
specific repository inclusions {and exclusions} (jlent)

It was possible to override the sshd_config file only with a complete
file.  This change allows the sshd_config file to be specified as a
template.

Correct template names for the impstats fragments that support
debugging rsyslog problems.  Update the documentation in the
base::syslog::fragment to make debugging a bit easier.

Conform to naming conventions for host base wallet objects.