acloud: default autoconnect to webrtc.

1. change default create argument as webrtc instead of vnc.
2. remove the tag appstreming due to the gce enforcer request the required port https://critique-ng.corp.google.com/cl/350100341.

Bug: 194363961
Bug: 200971710
Test: acloud-dev create
acloud-dev create --local-instance
acloud-dev create --autoconnect vnc

Change-Id: Ia8631592e10f63365c8e919d6a9afb77a6067c20
7 files changed