Terraform plugin private IP and logicalID allocation

[ndegory]

Fixes #393

When using logical ID allocations, if the private_ip property of the terraform plugin is set to INSTANCE_LOGICAL_ID, the property is replaced with the logical ID. Only enabled for AWS.

Usage example:

  {
    "Plugin": "group",
    "Properties": {
      "ID": "swarm-manager",
      "Properties": {
        "Allocation": {
          "LogicalIDs": [
            "192.168.2.200",
            "192.168.2.201",
            "192.168.2.202"
          ]
        },
        "Instance": {
          "Plugin": "instance-terraform",
          "Properties": {
            "Type": "aws_instance",
            "Value": {
              "ami": "${lookup(var.aws_amis, var.aws_region)}",
              "instance_type": "${var.aws_instance_type}",
              "key_name": "${var.aws_key_name}",
              "vpc_security_group_ids": ["${aws_security_group.default.id}"],
              "subnet_id": "${aws_subnet.default.id}",
              "private_ip": "INSTANCE_LOGICAL_ID"
              },
              "connection": {
                "user": "${var.aws_connection_user}"
              }
            }
          }
        },
        ...

in conjonction with the swarm flavor plugin.

Needs PR #392 to be applied first.

[ndegory]

@chungers thank you for merging #392, I can rebase this PR on master and force push it if it helps, please tell me.

[codecov-io]

Codecov Report

Merging #395 into master will decrease coverage by -0.01%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
- Coverage   62.57%   62.57%   -0.01%     
==========================================
  Files          56       56              
  Lines        3129     3137       +8     
==========================================
+ Hits         1958     1963       +5     
- Misses        944      945       +1     
- Partials      227      229       +2

Impacted Files	Coverage Δ
examples/instance/terraform/plugin.go	51.45% <62.5%> (+0.29%)	✅

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[ndegory]

rebased on master

[chungers]

@ndegory - thanks - nice work!