Automatic merge from submit-queue

Add SIG to test owners

**What this PR does / why we need it**:
This PR adds a `sig` column to the test owners file generation script.

A problem experienced with the current owners file is that since members are auto-assigned there are times where tests are assigned to non-active users who don't follow up to notifications to fix flakes. By assigning a SIG to each test we can hold a group we know is active responsible for taking care of flakes it's less likely that flakes will fall through the cracks.

**Special notes for your reviewer**:
* A companion PR will go into *kubernetes/contrib* adding support for mungers parsing this new column.
   * Another PR in contrib will add labeling GitHub flake issues with the appropriate SIG
* Currently SIGs are not labeled, this will be added in another PR where SIG determinations can be discussed

@saad-ali @pwittrock