release start: Create branch and then add labels

The order of actions taken was to

  1) Add release labels into Pivotal Tracker.
  2) Create the release branch.

If, however, the branch could not be created, the labels stayed in
Pivotal Tracker, confusing the plugin later on.

This patch makes sure that the branch is created before the labels are
inserted into Pivotal Tracker.

Change-Id: Ieaa80b09b7587679b149886e51cf42cd94ae9126
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>