CI: Add "throttle" label and "skip_gate" capability

To make throttling by label fully active, the "throttle" option
has to be specified with a specific label.

You can now specify "skip_gate" in the Gerrit comments when you
do a +2 code review to tell Jenkins not to actually run the
gate.  You'd do this if you plan to manually merge the change.

Also updated the "printenv" debug output to better sort multi-line
comments.

Change-Id: I4c0b1085acec4805f2ca207eebac50aad81f27e2