Rework the PUSH_IF_SUCCEED and CREATE_PR parameters in the T&P jobs

Currently the fact of using 2 boolean parameters is confusing, as there are 4 combinations but only 3 available behaviors:

1. PUSH_IF_SUCCEED && CREATE_PR => create PR
2. PUSH_IF_SUCCEED && !CREATE_PR => push
3. !PUSH_IF_SUCCEED && !CREATE_PR => nothing
4. !PUSH_IF_SUCCEED && CREATE_PR => nothing, so same as 3., but some people might think that it will create a PR

Suggestion: replace these 2 parameters by a single one ON_SUCCESS using a dropdown list with 3 choices:

1. Create PR => same as current 1.
2. Push => same as current 2.
3. Dry run / status check => same as current 3. and 4.

As a reminder, here is the current pseudo-code:

if [ "$MERGE_BEFORE_BUILD" != true ]; then 
  # merge
fi

if [ "$PUSH_IF_SUCCEED" = true ]; then
  if [ "$MERGE_BEFORE_BUILD" = true ]; then
    # pull
  fi
  if $CREATE_PR; then
    # PR
  else
    # Push
  fi
fi