Adding new metadata_from_file option to GCE module

Introducing a new option to allow users to set metadata values from the contents of files.  This is useful for taking advantage of passing in 'startup-scripts' [https://cloud.google.com/compute/docs/howtos/startupscript] to new instances.  An example playbook might look like,

```
- name: Create an instance with a startup-script
  hosts: local
  gather_facts: False
  vars:
    pid: MY_PROJECT
    email: hidden@developer.gserviceaccount.com
    pem: /home/erjohnso/pkey.pem
    machine_type: n1-standard-1
    image: debian-7
    itags: apache
    izone: us-central1-b
  tasks:
    - name: Bring up new instance with startup script
      gce:
        name: "metadata-test"
        metadata: '{"k1":"v1"}'
        metadata_from_file: '{"startup-script":"/home/erjohnso/bootup.sh"}'
        machine_type: "{{ machine_type }}"
        image: "{{ image }}"
        tags: "{{ itags }}"
        zone: "{{ izone }}"
        project_id: "{{ pid }}"
        pem_file: "{{ pem }}"
        service_account_email: "{{ email }}"
```

When the instance is created, it looks by default for a special metadata tag called 'startup-script'.  If that exists, the value is written to the instance's disk and executed automatically at instance creation.  Typical uses are initial configuration such as 'apt-get update'.