Use controlled execution environment, to avoid failure if PATH is unset (LP: #1959570)

[slyon]

Description

Use a controlled execution environment.
This is to allow execution in constrained environments, like early boot or
Ubuntu Core 20/22 where the PATH environment variable might not be set

Also, handle nmcli's "NetworkManager not running" error gracefully.
Considering /usr/bin/nmcli for normal NetworkManager and /snap/bin/nmcli -> /snap/bin/network-manager.nmcli for NetworkManager snap as valid implementations.

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad. LP#1959570

[slyon]

This fix seems to be working generally. There's still a brief interruption of the network connectivity, while restarting the "snap.network-manager.networkmanager.service" daemon (obviously), but the SSH connection is recovered after a few seconds.

I've built a core22 base snap, using https://launchpad.net/~slyon/+archive/ubuntu/lp1959570 and a UC22 image out of this to run this test (all executed over an SSH connection):

slyon@ubuntu:~$ sudo snap set system system.network.netplan.network.bridges.br54.dhcp4=true
slyon@ubuntu:~$ sudo netplan get
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    enp0s2:
      dhcp4: true
  bridges:
    br54:
      dhcp4: true
slyon@ubuntu:~$ sudo snap set system system.network.netplan.network.bridges.br54.dhcp4=false
slyon@ubuntu:~$ sudo netplan get
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    enp0s2:
      dhcp4: true
  bridges:
    br54:
      dhcp4: false

[daniloegea]

Would it be enough settings the environment variables only once via os.environ in the entry point of the python code?

[ogayot]

LGTM

[daniloegea]

lgtm

[slyon]

All tests green.

I've re-built a package in the above-mentioned PPA + core22 base snap + UC22 image and run the reproducer from the bug report:
=> It's still working as expected with the new os.environ.update() approach.

Merging.