Use `#check_for_command` instead of `#check_for_package`

where appropriate in `#install_puppet`

Previously there was no `Host#check_for_command` method, but `Host#check_for_package`
did a `which` (it effectively was checking for the command). Now that we've
separated out that behavior we need to use the ones we mean in
`InstallUtils#install_puppet()`. This is important because when we fall back
to a gem install of Puppet we ensure there is a command `gem` available from
(often) the package `rubygems`.