t/master: use_node_name is not a bool anymore

commit dd4cbb501ca7152abbc3687d461683b40c86435a added "ignore" as value
for use_node_name
master: fix partial graph_order handling

commit 53f22440a6809d8e4614ef46dfa8319a52f82bc7 fixed failures on
partial graph_order, but blindly added all the fields to it.

It should not be an issue as "" cleaned it upon use.

This commit fixes it beforehand, so noone else needs to be extra
careful, and tests don't fail...
t/spoolreader: remove unused File::Slurp dep
t/spoolwriter: explicit use read_file from File::Slurp
master/graph: fix warning on = in conditional
travis: We do _not_ want to load RRDs.pm