KCC: add comments, idiomatic changes to intrasite_graph code

KCC: add comments, idiomatic changes to intrasite_graph code

Using `x in foo_dict.keys()` is the same as `x in foo_dict`, except it
is O(n) instead of O(1) and is not the way things are done in Python.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>