network manager: do not store plaintext wifi passwords

[jaxxzer]

wifi psks should be produced by wpa_passphrase so that plaintext passwords are not saved.

[jaxxzer]

This should be pretty straightforward to implement: http://docs.ros.org/diamondback/api/wpa_supplicant/html/wpa__passphrase_8c_source.html

there are some python libraries to do the hash like hashlib and passlib: https://stackoverflow.com/questions/46502224/python-wpa-passphrase-linux-binary-implementation-generates-only-part-of-the-p