NetworkManager DNS configurator errors out when conf.d directory is missing instead of creating it #136

Describe the Bug

conf.d is not created on all systems, causing olm to fail initializing NetworkManager

Environment

• OS Type & Version: Unix, Void Linux, 6.18.41_1
• Pangolin cli Version: 0.15.1

To Reproduce

• ensure NetworkManager is running and detected (e.g. nmcli works, dbus reachable). Otherwise olm falls back to file-based DNS before ever reaching this code path
• remove /etc/NetworkManager/conf.d
• run pangolin up
• /etc/resolv.conf will be overwritten as a fallback, which can get overwritten by NetworkManager later on

Expected Behavior

• create the conf.d directory
• create olm-dns.conf inside it