PR description

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Submitter checklist

this pr implements the is default flag on network level
Api Doc: https://docs.openstack.org/api-ref/network/v2/index.html?expanded=list-networks-detail,show-network-details-detail#networks
Make sure that following is addressed to make the PR easier to process:

• [] If applicable, changes are covered by either a unit or a functional test.
  I get a ConcurrentModificationException when running the tests.
• If applicable, changes ware verified manually against an OpenStack instance.
• If the change is API related, the PR description links to OpenStack API documentation of that particular endpoint/request (https://docs.openstack.org/api-quick-start/#current-api-versions).
• If the change concerns particular OpenStack service, its name is used as a PR name prefix (ex.: "Neutron: Add floatingIp port forwardings service").
• If the PR closes existing GitHub issue, PR name have prefix: Fix #NNN: .