Shift WLAN passphrase to separate resource

[akerl]

I'm wondering if it's viable to shift the passphrase attribute to a separate resource from unifi_wlan, which would allow for configuring the WLAN w/o any sensitive information. Users who want to set a passphrase via code could do it via the new unifi_wlan_passphrase attribute, but for users that don't want the passphrase in their state file, they could deal with it externally.