Replace GCP firewall implementation with firecloud/cloud datastore

GCP does not have a direct analogue to firewalls/security group. A firewall resource in GCP is closer to a firewall rule in other providers. Therefore, the current implementation simulates groups of rules by a complicated procedure of tagging.

However, this complicates the code greatly, and has a significant impact on performance, since firewall rules must be repeatedly queried and grouped by tag to reconstruct a firewall. Therefore, an option is to use [Firestore database](https://firebase.google.com/docs/firestore/) as a simple means of storing the firewall -> firewall rule associations, so they can be queried faster. This will reduce complexity and result in improved performance.