Adding GCS as a policy-library store for on-GKE

[kevensen]

1. Removed the default policy_library_repository_url
2. Added policy_library_sync_enabled for on_gke examples
3. Added Workload Identity for config-validator
4. Corrected some Helm variables per the config-validator chart
5. on-GCE generates a private SSH key. on-GKE also behaves the same if a private ssh key file is not provided.

Tests:

• on_gke_end_to_end - config-validator disabled && git-sync disabled --> forseti deploys and config validator does not, no keys are created
• on_gke_end_to_end - config-validator disabled && git-sync enabled --> forseti deploys and config validator does not, no keys are created
• on_gke_end_to_end - config-validator enabled && git-sync disabled --> forseti deploys && config validator deploys && policy is pulled from GCS && no keys created
• on_gke_end_to_end - config-validator enabled && git-sync disabled && policy_library_url --> forseti deploys && config validator deploys && policy is pulled from Git && no keys created
• on_gke_end_to_end - config-validator enabled && git-sync enabled --> forseti deploys && config validator deploys && SSH key is generated
• on_gke_end_to_end - config-validator enabled && git-sync enabled && SSH key filename is provided--> forseti deploys && config validator deploys && public ssh-key output matches existing public key.
• on_gke_end_to_end [5.0.0 --> 5.1.0] - With config-validator enabled (and git-sync) and upgrade to 5.1.0(?) maintains functionality. This is the periodic config-validator.

[kevensen]

Addresses #341

[kevensen]

Need to test

• on_gke example with git-sync enabled. Then update policies and ensure pod deletes and restarts.

[gkowalski-google]

LGTM. Please create a ticket to support branch configuration on GCE.

[gkowalski-google]

Currently the GCE behavior is hard-coded to master branch. It's fine to support different branches, but if we do it for GKE, then we should do the same for GCE. That would mean a change here: https://github.com/forseti-security/forseti-security/blob/master/install/gcp/scripts/initialize_forseti_services.sh#L55

[kevensen]

Opened #366