Add Priority Setting

[TheNageek]

I'm relatively new to connman, but it's actively being used on a project I'm working on. We're coming across an issue where we'd like to prioritize one type of connection over another. A supplicant, e.g. wpa_supplicant, who performs the authentication with an access point does have a priority parameter. Thus it would be great if connman also had a priority parameter that could be passed down to the supplicant.