Add reviewkeys

Reviewkeys are a customizable shortcut for leaving predefined
votes on a change.  For example, if you want to leave a +2
review with no comment, you might simply hit the "meta-2"
key.  The following configuration block is recommended for
use with OpenStack's Gerrit:

  reviewkeys:
    - key: 'meta 0'
      approvals: []
    - key: 'meta 1'
      approvals:
        - category: 'Code-Review'
          value: 1
    - key: 'meta 2'
      approvals:
        - category: 'Code-Review'
          value: 2
    - key: 'meta 3'
      approvals:
        - category: 'Code-Review'
          value: 2
        - category: 'Workflow'
          value: 1

Change-Id: I466024d71d5b1d89fcdc32355e2a9fe13abf5c28