The `boto3_conn` function requires a module argument, and calls
`module.fail_json` if the connection doesn't receive enough arguments.
In non-module settings like inventory scripts, there is no module to be
passed.

The `boto3_inventory_conn` function takes the same arguments except for
`module`, and both call _boto3_conn which doesn't require a module be
passed.

Add db_clusters to the ec2 inventory. Show tags. Only show clusters
matching tags in the `.ini`. Set `include_rds_clusters = True` option to
enable RDS cluster inventory collection.

Example inventory output:

```
{
  "db_clusters": {
    "ryansb-cluster-test": {
      "AllocatedStorage": 1,
      "AvailabilityZones": [
        "us-west-2a",
        "us-west-2b",
        "us-west-2c"
      ],
      "BackupRetentionPeriod": 1,
      "DBClusterIdentifier": "ryansb-cluster-test",
      "DBClusterMembers": [
        {
          "DBClusterParameterGroupStatus": "in-sync",
          "DBInstanceIdentifier": "ryansb-test",
          "IsClusterWriter": true,
          "PromotionTier": 1
        },
        {
          "DBClusterParameterGroupStatus": "in-sync",
          "DBInstanceIdentifier": "ryansb-test-us-west-2b",
          "IsClusterWriter": false,
          "PromotionTier": 1
        }
      ],
      "DBClusterParameterGroup": "default.aurora5.6",
      "DBSubnetGroup": "default",
      "DatabaseName": "mydb",
      "DbClusterResourceId": "cluster-OB6H7JQURFKFD4BYNHG5HSRLBA",
      "Endpoint": "ryansb-cluster-test.cluster-c9ntgaejgqln.us-west-2.rds.amazonaws.com",
      "Engine": "aurora",
      "EngineVersion": "5.6.10a",
      "MasterUsername": "admin",
      "Port": 3306,
      "PreferredBackupWindow": "06:09-06:39",
      "PreferredMaintenanceWindow": "mon:11:22-mon:11:52",
      "ReadReplicaIdentifiers": [],
      "Status": "available",
      "StorageEncrypted": false,
      "VpcSecurityGroups": [
        {
          "Status": "active",
          "VpcSecurityGroupId": "sg-47eaea20"
        }
      ]
    }
  },
  "rds": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "rds_aurora": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "rds_parameter_group_default_aurora5_6": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "ryansb-test": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "ryansb-test-us-west-2b": [
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "type_db_r3_large": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "us-west-2": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "us-west-2a": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "us-west-2b": [
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ],
  "vpc_id_vpc_3ca34459": [
    "ryansb_test_c9ntgaejgqln_us_west_2_rds_amazonaws_com",
    "ryansb_test_us_west_2b_c9ntgaejgqln_us_west_2_rds_amazonaws_com"
  ]
}
```

Updated as per @ryansb comments. The EC2 inventory script will now fail
with a useful message when boto3 is not installed and the user is trying
to read RDS cluster information.

Using boto3 directly wasn't properly using profiles set in the `ec2.ini`
file, this change uses the `module_utils` boto3_conn instead.