Fixes 4775: add weekly cron for pulp orphan cleanup

[rverdile]

Summary

Adds a new command and weekly cron job to run orphan cleanup for pulp. Runs in batches of 5 orgs.

Testing steps

1. You can use this script to create a bunch of orgs

TOTAL_ORGS=33

for ((i=0; i<$TOTAL_ORGS; ++i))
do

ORG_ID=`tr -dc A-Za-z0-9 </dev/urandom | head -c 13`

HEADER=`./scripts/header.sh $ORG_ID`

UUID=$(curl -X POST --location "http://localhost:8000/api/content-sources/v1.0/repositories/" \
    -H "${HEADER}" \
    -H "Content-Type: application/json" \
    -d '{
          "name": "comps repo 2",
          "url": "https://rverdile.fedorapeople.org/dummy-repos/comps/repo2/",
          "snapshot": true
  }' | jq -r .uuid)

curl -X DELETE --location "http://localhost:8000/api/content-sources/v1.0/repositories/$UUID" \
    -H "${HEADER}" \
    -H "Content-Type: application/json"
done

2. Run go cmd/external-repos/main.go pulp-orphan-cleanup
3. In the logs you'll see the pulp tasks running in groups of 5

[jlsherrill]

https://issues.redhat.com/browse/HMS-4775