Make node_exporter.service template compatible with both Python 2 and 3

[nikosgraser]

When using this role in an environment where the default python
interpreter is python3, the ansible run fails when jinja2 tries to
process templates/node_exporter.service.j2.

This is because in Python 3, dict.items() returns a view object
which can't be indexed directly (in Python 2, this returns a list).

Casting the result of c.items() explicitly to a list allows indexing
under Python 3 while not changing the outcome in Python 2.

When executed in a Python 3 environment, the order
in which the items are looped over might be different since "Keys and
values are listed in an arbitrary order which is non-random, varies
across Python implementations, and depends on the dictionary’s history
of insertions and deletions."
(https://docs.python.org/3.6/library/stdtypes.html#dict-views)

Otherwise, this change is functionally identical.

[SuperQ]

LGTM

[lock]

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