net_if_management.py returns networkd rather than NetworkManager on Xenial desktop

[Taihsiang Ho]

net_if_management.py returns networkd rather than NetworkManager on Xenial desktop. This will result in that all wifi test cases could not be activated expectedly.

[Steps to Reproduce]

1. Install Xenial Desktop
2. Install checkbox from dev ppa
3. execute /usr/lib/plainbox-provider-resource-generic/bin/net_if_management.py

[Expected Result]

device: wlp2s0
nmcli_available: True
managed_by: NetworkManager

[Actual Result]

device: wlp2s0
nmcli_available: True
managed_by: networkd

[More Information]

There is no /etc/netplan in Xenial Desktop

[Package Information]

ii checkbox-converged 1.7.0+git201802201240+pkg15~ub all testing tool for all Ubuntu devices
ii checkbox-gui 1.2.4-0ubuntu1 all QML based interface for checkbox (transitional package)
ii checkbox-ng 1.4.0+git201907030956+pkg115~u all CheckBoxNG test runner
un plainbox <none> <none> (no description available)
ii plainbox-insecure-policy 1.4.0+git201907030956+pkg115~u all policykit policy required to use CheckBoxNG (insecure version)
ii plainbox-provider-checkbox 0.48.0+git201907041516+pkg107~ amd64 CheckBox provider for PlainBox
ii plainbox-provider-resource-generic 0.40.0+git201906271030+pkg71~u amd64 CheckBox generic resource jobs provider
ii plainbox-provider-sru 1.13.0+git201901241016+pkg39~u all CheckBox SRU jobs provider
un plainbox-secure-policy <none> <none> (no description available)
ii python3-checkbox-ng 1.4.0+git201907030956+pkg115~u all CheckBoxNG test runner (Python 3 library)
ii python3-checkbox-support 0.41.0+git201907030755+pkg95~u all collection of Python modules used by PlainBox providers
un python3-plainbox <none> <none> (no description available)