Remove has_variable from templates

Without this patch the use of has_variable? in templates are causing
logic errors.  PXE configuration options are being added when they
should not be.

This patch replaces has_variable? with instance variables.  This fixes
the problem and correctly produces templates.

NOTE: The concat module assumes an empty string passed to content is the
same as content being undefined.  As a result, the template fragment
uses a static header comment to always make sure the string is not
empty.

templates/dhcpd.conf-extra.erb

@@ -1,6 +1,6 @@
 # ----------
 # Extra Configs
 # ----------
-include "<%= dhcp_dir %>/dhcpd.pools";
-include "<%= dhcp_dir %>/dhcpd.hosts";
+include "<%= @dhcp_dir %>/dhcpd.pools";
+include "<%= @dhcp_dir %>/dhcpd.hosts";
 

templates/dhcpd.conf.pxe.erb

@@ -1,8 +1,6 @@
-<% if has_variable?( 'pxeserver' ) and has_variable?( 'pxefilename' ) then -%>
-# ----------
-# PXE Handoff
-# ----------
-next-server <%= pxeserver %>;
-filename "<%= pxefilename %>";
+# BEGIN PXE Section
+<% if @pxeserver and @pxefilename then -%>
+next-server <%= @pxeserver %>;
+filename "<%= @pxefilename %>";
 <% end -%>
-
+# END PXE Section