Enable a uuid flavor

Current tempest configuration enforces a flavor id should be integer,
but we can create a uuid flavor. For example, the following command
creates a flavor with auto-generated uuid:

  $ nova flavor-create m3.small auto 512 0 1

This patch enables a uuid flavor by changing a flavor_id type from
integer to string.

Change-Id: I6567ef3086710508ef4215e93634601119ce3402
Merge "Enable a uuid flavor"