Add alpine based dockerfile for a smaller container image

[gunjan5]

This reduces the image size by 75%

https://hub.docker.com/r/gunjan5/terraform_ansible/tags/

It does the following:

• Alpine 3.6 base image
• Install all the dependencies (doesn't need ./home/pan_install.sh)
• Install Ansible
• Install Terraform 0.11.7 (latest)
• Clone ansible-pan repo
• Copy terraform-templates (form this repo)
• install PaloAltoNetworks modules from Ansible Galaxy
• Terraform initialize one-click install templates for AWS and Azure (download the terraform modules)

I've pushed the image to Dockerhub at gunjan5/terraform_ansible:latest

[gunjan5]

@vinayvenkat Thanks! I've added the Readme and moved it to automation_container folder. PTAL. I can update the main readme once I have the command/argument CLI done (in the next PR)