curtin.net: emit source /etc/network/interfaces.d/*.cfg in eni
- Play nice with users and have eni source interfaces.d configurations.
- Fix up network unittests
- Remove restriction in curtin's eni parser about iface redefinition;
man (5) interfaces says this is allowed.
- Update output of render_interfaces to account for source line
- Add vmtest to test user-defined network config in interfaces.d