don't require the host to be in the Image tag

now that we know our base host via config, we can stop hardcoding it in
each Image tag

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

tests/instance/pages/more-metadata.md

@@ -1,6 +1,6 @@
 Title:          title for the page
 Description:    description of this page
                 made even longer
-Image:          http://buh.com/test.img
+Image:          /test.img
 
 hello