Bug 2105 fixed

- Added the --reuse-values flag to the upgrade command and updated the
  tests accordingly;
- Also, removed the duplicated code from the _get_upgrade_scale_command
  method, and not this method only calls the _get_upgrade_command to get
  the command to upgrade/scale the Helm Chart

Change-Id: I8bda5450f1f9bd5e2b9771f284e1b904b82fb3df
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>