Updated module to be compliant with test cases.

* Added integration tests
* Added unit tests

Originally from ansible/ansible-modules-extras@ee523be26c

test/integrations/group_vars/all.yml

@@ -0,0 +1 @@
+test_subnet_id: 'subnet-123456789'

test/integrations/roles/ec2_vpc_nat_gateway/tasks/main.yml

@@ -0,0 +1,76 @@
+- name: Launching NAT Gateway and allocate a new eip.
+  ec2_vpc_nat_gateway:
+    region: us-west-2
+    state: present
+    subnet_id: "{{ test_subnet_id }}"
+    wait: yes
+    wait_timeout: 600
+  register: nat
+
+- debug:
+    var: nat
+- fail:
+    msg: "Failed to create"
+  when: '"{{ nat["changed"] }}" != "True"'
+
+- name: Launch a new gateway only if one does not exist already in this subnet.
+  ec2_vpc_nat_gateway:
+    if_exist_do_not_create: yes
+    region: us-west-2
+    state: present
+    subnet_id: "{{ test_subnet_id }}"
+    wait: yes
+    wait_timeout: 600
+  register: nat_idempotent
+
+- debug:
+    var: nat_idempotent
+- fail:
+    msg: "Failed to be idempotent"
+  when: '"{{ nat_idempotent["changed"] }}" == "True"'
+
+- name: Launching NAT Gateway and allocate a new eip even if one already exists in the subnet.
+  ec2_vpc_nat_gateway:
+    region: us-west-2
+    state: present
+    subnet_id: "{{ test_subnet_id }}"
+    wait: yes
+    wait_timeout: 600
+  register: new_nat
+
+- debug:
+    var: new_nat
+- fail:
+    msg: "Failed to create"
+  when: '"{{ new_nat["changed"] }}" != "True"'
+
+- name: Launching NAT Gateway with allocation id, this call is idempotent and will not create anything.
+  ec2_vpc_nat_gateway:
+    allocation_id: eipalloc-1234567
+    region: us-west-2
+    state: present
+    subnet_id: "{{ test_subnet_id }}"
+    wait: yes
+    wait_timeout: 600
+  register: nat_with_eipalloc
+
+- debug:
+    var: nat_with_eipalloc
+- fail:
+    msg: 'Failed to be idempotent.'
+  when: '"{{ nat_with_eipalloc["changed"]  }}" == "True"'
+
+- name: Delete the 1st nat gateway and do not wait for it to finish
+  ec2_vpc_nat_gateway:
+    region: us-west-2
+    nat_gateway_id: "{{ nat.nat_gateway_id }}"
+    state: absent
+
+- name: Delete the nat_with_eipalloc and release the eip
+  ec2_vpc_nat_gateway:
+    region: us-west-2
+    nat_gateway_id: "{{ new_nat.nat_gateway_id }}"
+    release_eip: yes
+    state: absent
+    wait: yes
+    wait_timeout: 600