(#13860) Make a2enmod/a2dismo commands optional

Previous to this comit, the a2enmod and a2dismod commands were required
for the a2mod provider to be used by puppet on the first run.  Since
these commands are not always available on the first run, the catalog
application might fail.  This commit makes the commands optional, so the
provider's validity will be evaluated when an a2mod resource is enforced

lib/puppet/provider/a2mod/a2mod.rb

@@ -1,21 +1,21 @@
 Puppet::Type.type(:a2mod).provide(:a2mod) do
     desc "Manage Apache 2 modules on Debian and Ubuntu"
- 
+
-    commands :encmd => "a2enmod"
+    optional_commands :encmd => "a2enmod"
-    commands :discmd => "a2dismod"
+    optional_commands :discmd => "a2dismod"
- 
+
     defaultfor :operatingsystem => [:debian, :ubuntu]
 
     def create
         encmd resource[:name]
     end
- 
+
     def destroy
         discmd resource[:name]
     end
- 
+
     def exists?
-        mod= "/etc/apache2/mods-enabled/" + resource[:name] + ".load" 
+        mod= "/etc/apache2/mods-enabled/" + resource[:name] + ".load"
         File.exists?(mod)
     end
 end