Refs #4464 - Prefer selinux wrappers

This is needed to start "puppet ca" command in the correct puppetca_t domain.

RHEL6 has `/usr/bin/puppetca`

RHEL7 currently has no such wrapper and I will be pushing fix to EPEL7 and
Fedora with wrapper (they are named start-puppet-xyz so I will follow this
convention)

Puppet Enterprise is a question mark for now since there are no wrappers - no
SELinux support?
the new items in the default_path are executables, correct? If so, this won't work, as 'which' expects a list of directories as the second parameter.