fix for #802: when !manage_docroot, don't require it

in #802 we introduced manage_docroot, however, I overlooked a place in
the code were we `require` it. This patch moves the priority_real, and
filename evaluation above, so we can simply use a `before`.

Perhaps we should cluster all variable transformation right below the
validation code?
Merge pull request #806 from igalic/manage-docroot

fix for #802: when !manage_docroot, don't require it