netplan apply crashes if ovs service is masked

[George Shuklin]

If OVS is installed but masked `netplan apply` crashes even if there is no OVS-related configuration present.

Steps to reproduce:
1. apt-get install openvswitch-common openvswitch-switch
2. systemctl mask ovs-vswitchd.service ovsdb-server.service
3. netplan apply.

Expected behavior: work normal

Actual behavior:

netplan apply
ovs-vsctl: unix:/var/run/openvswitch/db.sock: database connection failed (No such file or directory)
Traceback (most recent call last):
  File "/usr/sbin/netplan", line 23, in <module>
    netplan.main()
  File "/usr/share/netplan/netplan/cli/core.py", line 50, in main
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 310, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 59, in run
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 310, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 165, in command_apply
    NetplanApply.process_ovs_cleanup(config_manager, old_files_ovs, restart_ovs, exit_on_error)
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 371, in process_ovs_cleanup
    apply_ovs_cleanup(config_manager, ovs_old, ovs_current)
  File "/usr/share/netplan/netplan/cli/ovs.py", line 138, in apply_ovs_cleanup
    out = subprocess.check_output([OPENVSWITCH_OVS_VSCTL, '--columns=name,external-ids',
  File "/usr/lib/python3.8/subprocess.py", line 415, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/usr/bin/ovs-vsctl', '--columns=name,external-ids', '-f', 'csv', '-d', 'bare', '--no-headings', 'list', 'Port']' returned non-zero exit status 1.

Ubuntu version: 20.04