Add ELB-Registration to EC2 Template

[ferricoxide]

Is your feature request related to a problem? Please describe.

Current automation relies on either manual registration of EC2 instance to the ELB or an action within the supplied Jenkins pipeline-job. In the later case, there is a race-condition wherein the ELB registration-attempt may happen when either the ELB or the EC2 are not sufficiently ready, causing the EC2 Jenkins job to fail

Describe the solution you'd like

1. Add registration-step to EC2-level automation (something like):

   {
     "Fn::If": [
       "AttachToElb",
       {
         "Fn::Join": [
           "",
           [
             "# Register to Named ELB\n",
             "aws elb --region ",
             { "Ref": "AWS::Region" },
             " register-instances-with-load-balancer --load-balancer-name ",
             { "Ref": "ElbName" },
             " --instances ",
             " $( curl -skl http://169.254.169.254/latest/meta-data/instance-id/ )",
             "\n"
           ]
         ]
       },
       ""
     ]
   },

   to the end of the EC2 template's UserData's script.sh section. Add ElbName parameter{} and AttachToElb condition{} as appropriate
2. Remove (presumably) redundant registration-step from Jenkins pipeline-job.

Describe alternatives you've considered

If converting to use with ELBv2 (ALB), implement by adding reference to ALB's Tgroup.