Add new arg "--gce-metadata" to record extra data for GCE instance.

Acloud would record the extra data into GCE metadata with key value pair
format. e.g. testid:11,retry:3

Bug: 177709191
Bug: 178510051
Test: acloud-dev create --gce-metadata testid:11,retry:3
Change-Id: I0b5a0e78b15017c8828ba8dca2e9e3a5cd720b32
5 files changed