Increase default openstack flavor for management cluster

Francois Eleouetrequested to merge
increase-mgmt-default-flavor into main

What does this MR do and why?

as m1.large is not large-enough to host management cluster, move to a larger default flavor (m1.xlarge, the flavor that is used in CI)

Related reference(s)

relates to: #1409 (closed)

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