Tags feature: support tags specification for NAT gateway (solves ansi…

…ble-collections#345)

- implements tags feature for NAT gateway
- adds integration test tasks for tags feature
- refactor integration tests (overall) removing hard-coded parameters
- add missing integration test tasks without CHECK_MODE

plugins/modules/ec2_vpc_nat_gateway.py

@@ -59,7 +59,7 @@
       - Remove tags not listed in I(tags).
     type: bool
     default: true
-    version_added: 1.4
+    version_added: 1.4.0
   release_eip:
     description:
       - Deallocate the EIP from the VPC.