Fix service image boot issues
Fix service image boot issues

We are getting:

403 errors 'Only volume-backed servers are allowed for flavors
with zero disk' on scenario tests.

Appears to be due to this change [1] which just merged to
nova master branch.  Ubuntu bionic server doc says the root disk
needs 1.5 GB so we are setting the flavor definition to require
2GB.

[1] https://github.com/openstack/nova/commit/c8e65a5eb11515cfe70f8e6850b842cd594af6a5

Closes-bug: #1816050
Change-Id: Iba0a15b78bf75a04c1ac0e64e70634772b2dca5c

openstack/manila,openstack/manila