Move DB creation from st2mistral to postgresql role

[arm4b]

Since we have own postgresql role, - let it handle DB creation for st2mistral.

Use native Ansible modules postgresql_user and postgresql_db instead of init_mistral_db SQL template file.

So after all postgresql + st2mistral usage should look as following:

- name: Install st2mistral on a single node
  hosts: all
  roles:
    - name: Install PostgreSQL and configure databases
      role: postgresql
      vars:
        postgresql_databases:
          - name: "{{ st2mistral_db }}"
        postgresql_users:
          - name: "{{ st2mistral_db_username }}"
            pass: "{{ st2mistral_db_password }}"
            encrypted: yes

    - name: Install and configure StackStorm Mistral
      role: st2mistral
      vars:
        st2mistral_database_connection: postgresql://{{ st2mistral_db_username }}:{{ st2mistral_db_password }}@localhost/{{ st2mistral_db }}

Use best practices from https://github.com/ANXS/postgresql

Additionally, this will allow us to manage multi-node deployments #17 better in future.

[arm4b]

This issue is the only blocker now to allow StackStorm installations with external Mongo, RabbitMQ, PostgreSQL #17

[cognifloyd]

I would rather see the stackstorm and mistral specific features included in st2 specific roles so that there is still the possibility of either using an external postgres role (say, in a custom playbook) or reusing stackstorm's postgres role in external playbooks (ie in non-stackstorm playbooks).

Same goes for mongo and rabbitmq. I want to use the same roles to install those tools across my infrastructure (to maintain consistent variables across my inventory) and then add additional roles to setup app-specific databases/collections/queues after those services are installed.