Add default for number of executors for a new computer

Signed-off-by: Kienan Stewart <kstewart@efficios.com>

Author: kienanstewart

src/main/resources/hudson/plugins/libvirt/VirtualMachineSlave/configure-entries.jelly

@@ -50,7 +50,7 @@
     </f:entry>
 
     <f:entry title="${%# of executors}" field="numExecutors">
-        <f:textbox checkMethod="post"/>
+        <f:textbox checkMethod="post" default="1"/>
     </f:entry>
 
     <f:entry title="${%Remote FS root}" field="remoteFS">