Fix config edit traceback on page menu.

[oliver-sanders]

Steps to replicate:

1. Open rose edit.
2. Select rose-suite.conf from the tree menu.
3. Open the page menu.

[matthewrmshin]

(Test failure due to change in Travis CI. Fix for the test already proposed in #2132.)

[matthewrmshin]

@sadielbartholomew please sanity check.

[sadielbartholomew]

Yes, I get the traceback:

Traceback (most recent call last):
  File " ... lib/python/rose/config_editor/main.py", line 601, in <lambda>
    self._get_current_page()
  File " ... lib/python/rose/config_editor/menu.py", line 483, in load_page_menu
    add_var_menu = current_page.get_add_menu()
  File " ... lib/python/rose/config_editor/page.py", line 579, in get_add_menu
    named_item.var_id = variable.metadata['id']
AttributeError: 'NoneType' object has no attribute 'var_id'

which disappears with the PR change which solves the error & from looking at the code there should not be any adverse consequences elsewhere either.