service.exists with systemd fails

[sblaisot]

On a systemd with systemd using SystemdService class, service.exists property fails with

 AssertionError: Unexpected exit code 2 for CommandResult(backend=<testinfra.backend.docker.DockerBackend object at 0x7f23a47a8520>, exit_status=2, command=b'systemctl list-unit-files | grep -q"^avahi-daemon"', _stdout=b'', _stderr=b"grep: invalid option -- '^'\nUsage: grep [OPTION]... PATTERNS [FILE]...\nTry 'grep --help' for more information.\n")
assert 2 in [0, 1]
+  where 2 = CommandResult(backend=<testinfra.backend.docker.DockerBackend object at 0x7f23a47a8520>, exit_status=2, command=b'syst...=b"grep: invalid option -- '^'\nUsage: grep [OPTION]... PATTERNS [FILE]...\nTry 'grep --help' for more information.\n").rc