Support Consul for distributed consensus

[vitabaks]

Add a new type of scheme: "[Type C] PostgreSQL High-Availability with Consul Service Discovery (DNS)" based on "Patroni" and "Consul".

Based on role: https://github.com/ansible-community/ansible-consul

issue #85

To test, follow these steps:

# 1. clone this repository from branch "consul"
git clone --branch consul --single-branch https://github.com/vitabaks/postgresql_cluster.git

# 2. Go to the playbook directory
cd postgresql_cluster/

# 3. Edit the inventory file
vim inventory
## Specify (non-public) IP addresses and connection settings (`ansible_user`, `ansible_ssh_pass` or `ansible_ssh_private_key_file` for your environment

# 4. Edit the variable file vars/main.yml
vim vars/main.yml
## Specify the variable: "dcs_type: consul"

# 5. install consul role requirements on the control node
ansible-galaxy install -r roles/consul/requirements.yml

# 6. Run playbook:
ansible-playbook deploy_pgcluster.yml

Client access point:

• master.postgres-cluster.service.consul
• replica.postgres-cluster.service.consul