fuel_upgrade: add retries for getting token

After starting a new Keystone container, the first attempt to get a token
fails. Unfortunately, the logs keep silent and say nothing. As a workaround,
we can just increase retries number.

Change-Id: Ib1bba120614d1964e838c289489f1877074c324d
Closes-Bug: #1403798
Merge "fuel_upgrade: add retries for getting token"