add mongodb-firewall role for iptables rules

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ansible/roles/mongodb-firewall/README.md

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+# Description
+
+This role simply open the TCP port in `iptables` based on value of `mongodb_net_port`.

ansible/roles/mongodb-firewall/defaults/main.yml

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+---
+mongodb_net_port: 27017

ansible/roles/mongodb-firewall/handlers/main.yml

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+---
+- name: Save iptables rules
+  shell: iptables-save > /etc/iptables/rules.v4

ansible/roles/mongodb-firewall/tasks/main.yml

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+---
+- name: Enable MongoDB port
+  iptables:
+    comment: 'MongoDB'
+    jump: ACCEPT
+    source: '0.0.0.0/0'
+    protocol: 'tcp'
+    destination_port: '{{ mongodb_net_port }}'
+  notify:
+    - Save iptables rules