You can override it for all devs of repo by adding line to codereview.settings:
  GERRIT_SQUASH_UPLOADS: (True|False)

And even with that, individual devs can overide it for themselves with:
  git config [--global] --bool gerrit.override-squash-uploads (true|false)

BUG=611892

Review-Url: https://codereview.chromium.org/2072363002