only ask for sudo password when required

[epheo]

Most desktop Linux distributions do not require sudo privileges to control networking.

nmcli takes its permission from Polkit /usr/share/polkit-1/actions/org.freedesktop.NetworkManager.policy

I'd like OpenGoPro Python SDK to only ask for sudo privileges when necessary.

One way to implement this would be to:

• check that the user has sufficient privileges using nmcli general permissions
• ask for sudo password if not

Attaching a PR.