Add basic support for SPOT instances in GCP

[bsda]

I've been running this fork for the last few months and it is working well.

I know there's some resistance against supporting SPOT instances, so feel free to drop this PR

The new env variable to enable it is DRONE_GOOGLE_PROVISIONING_MODEL: SPOT
It defaults to STANDARD if not specified

It seems to work well with the REAPER enabled. I've been running it with these options and it has saved us a ton of money:

DRONE_REAPER_ENABLED: true
DRONE_REAPER_INTERVAL: 10m
DRONE_PINGER_ENABLED: true
DRONE_PINGER_INTERVAL: 1m
DRONE_POOL_MAX: "10"
DRONE_POOL_MIN: "0"
DRONE_POOL_MIN_AGE: "30m"