Break up into 2 functions, simplify control flow, fix asserts, remove
unused functions.

fix: detection of the mapped dst network reworked again (to avoid creation of network when there is mapped one in admin project)

Fixes following tracebacks:
2024-07-09 13:41:13,648 project-migrator INFO F.3 Source server ostack port(s) detected
2024-07-09 13:41:13,648 project-migrator INFO F.3 Source server ostack single (unambiguous) port detected
Traceback (most recent call last):
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/project-migrator.py", line 402, in <module>
    sys.exit(main(ARGS))
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/project-migrator.py", line 173, in main
    olib.get_or_create_dst_server_networking(args,
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/olib.py", line 265, in get_or_create_dst_server_networking
    i_dst_network, _, _ = create_destination_networking(args,
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/olib.py", line 113, in create_destination_networking
    src_subnets = [ src_ostack_conn.network.find_subnet(i_src_subnet_id) for i_src_subnet_id in src_network.subnet_ids ]
AttributeError: 'NoneType' object has no attribute 'subnet_ids'

Depends on https://gitlab.ics.muni.cz/cloud/g2/prod-brno-cloud-entities/-/merge_requests/58.

This patch fixes following problem:
2024-07-02 12:29:49,117 project-migrator INFO F.9 Destination OpenStack server keypair exists (vscht)
Traceback (most recent call last):
  File "/home/freznicek/prjs/muni/sources/gitlab.ics.muni.cz/g1-g2-ostack-cloud-migration.git/ci/./project-migrator.py", line 389, in <module>
    sys.exit(main(ARGS))
             ^^^^^^^^^^
  File "/home/freznicek/prjs/muni/sources/gitlab.ics.muni.cz/g1-g2-ostack-cloud-migration.git/ci/./project-migrator.py", line 190, in main
    olib.get_or_create_dst_server_security_groups(args,
  File "/home/freznicek/prjs/muni/sources/gitlab.ics.muni.cz/g1-g2-ostack-cloud-migration.git/ci/olib.py", line 452, in get_or_create_dst_server_security_groups
    for i_src_server_security_group_name in {i_sg['name'] for i_sg in src_server.security_groups}:
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'NoneType' object is not iterable
[freznicek@lenovo-t14 ci 1]$