fix: more robust source cloud keypair selection

This MR introduces more robust source cloud keypair selection.

This change is triggered by situations like following

2024-06-19 17:07:24,875 project-migrator INFO F.6 Destination OpenStack flavor exists
Traceback (most recent call last):
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/project-migrator.py", line 389, in <module>
    sys.exit(main(ARGS))
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/project-migrator.py", line 184, in main
    i_destination_server_keypair = olib.get_or_create_dst_server_keypair(args, source_keypairs,
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/olib.py", line 342, in get_or_create_dst_server_keypair
    log_or_assert(args,
  File "/builds/cloud/g1-g2-ostack-cloud-migration/ci/lib.py", line 218, in log_or_assert
    assert condition, "\n".join([msg, msg_guidance]) if msg_guidance else msg
AssertionError: F.7 Source OpenStack server keypair found (jhandl).
Current source OpenStack cloud keypair dump is outdated already and does not contain mentioned keypair.Re-dump source OpenStack keypairs to ceph migrator server node and retry migration.

Looking into source keypair data it looks that keypair jhandl is present but under different user-id