Check nmcli permissions and only ask for password if required.

Move getpass to separate func and validate password input using assert
Otherwise raise AssertionError

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

As password isn't a mandatory requirement for the follwing classes
also accept None type arg for password.
- Nmcli0990Wireless
- NmcliWireless
- WpasupplicantWireless

Replace assert with if not raise RunTime error.