Ensure default flavor is set when upgrading tempest

Add DEFAULT_INSTANCE_TYPE to upgrade tempest to ensure this is
set. This avoids recreation of existing flavors when upgrading
tempest.

Change-Id: If2e872132939faa4b8afc8dc9b7d7d9aeb026c96
Fix: Bug 1178028
Merge "Ensure default flavor is set when upgrading tempest"