Only ask for sudo password when required

[epheo]

This resolves https://github.com/gopro/OpenGoPro/issues/483

Check nmcli permissions and only ask for password if required.

Move getpass to separate func and validate password input, Otherwise raise RuntimeError

Change sudo func to return an empty string if no sudo password is provided in order to prevent formatting of the nmcli commands with sudo prefix if not required. Raise condition is moved at password input using assert.