Swarming: Calculates CF zip url in preprocess to be used in main

[IvanBM18]

The setups scripts rely on a set of values that form the GS storage url to download the latest CF .zip, these values either come from the env vars, or the metadata server.

Since in swarming we don't have metadata server, we are calculating these values beforehand and sending them as env vars in the swarming request.

Test Proofs

These changes have been in dev since last week. We have successfully use them to fix this issue:

Downloading ClusterFuzz source code.
++ '[' '' = 1 ']'
++ gsutil cp gs:///linux-3.zip clusterfuzz_package.zip

Allowing the code to tgarget the correct URL:

Downloading ClusterFuzz source code.
++ '[' '' = 1 ']'
++ gsutil cp gs://deployment.clusterfuzz-development.appspot.com/linux-3.zip clusterfuzz_package.zip

Note: The failing e2e check is known to be failing right now so we can ignore that

[letitz]

LGTM modulo a couple comments below, to avoid blocking on me further.

[jardondiego]

LGTM