Added a modify-reviewers ssh command

Usage example:

  ssh -p 29418 review.example.com gerrit modify-reviewers \
    --project crypto Iac6b2ac2 \
    --add alice@example.com \
    --add bob@example.com \
    --remove eve@example.com

* You may specify the Change-Id as either the db id or an abbreviated
  change key.  If multiple changes are found due to ambiguity an
  error message is displayed.

* No message is displayed if the user tries to add a reviewer
  already on a change or remove a reviewer who is not on a change

Change-Id: Id1044e3f7d55415e26d334c2c73917dc72776fed
5 files changed