Change self.node to self.nodes in model_root

networkx removed G.node in version 2.4[1]
G.node was replaced by G.nodes since version 2.0[2],
and supports Python 2.7, 3.5, 3.6 and 3.7 from 2.2
so the lower constraint version is 2.2.
lib task_flow also invokes lib networkx,
task_flow version is also needed to be updated.
[1]: https://networkx.github.io/documentation/stable/release/release_2.4.html
[2]: https://networkx.github.io/documentation/stable/release/release_2.0.html
Change-Id: I268bcf57ec977bd8132a9f1573b28b681cb4ce1e
Closes-Bug: #1854132

lower-constraints.txt

@@ -64,7 +64,7 @@ msgpack==0.5.6
 munch==2.2.0
 netaddr==0.7.19
 netifaces==0.10.6
-networkx==1.11
+networkx==2.2
 openstackdocstheme==1.20.0
 openstacksdk==0.12.0
 os-api-ref===1.4.0
@@ -145,7 +145,7 @@ sqlparse==0.2.4
 statsd==3.2.2
 stestr==2.0.0
 stevedore==1.28.0
-taskflow==3.1.0
+taskflow==3.7.1
 Tempita==0.5.2
 tenacity==4.9.0
 testresources==2.0.1