mgr/cephadm: don't add mgr into iscsi trusted_ip_list if it's already there

[trociny]

Fixes: https://tracker.ceph.com/issues/58792

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[adk3798]

https://pulpito.ceph.com/adking-2023-03-06_16:09:09-orch:cephadm-wip-adk-testing-2023-03-04-1231-distro-default-smithi/

Failed/dead jobs reruns: https://pulpito.ceph.com/adking-2023-03-06_21:04:02-orch:cephadm-wip-adk-testing-2023-03-04-1231-distro-default-smithi/

After reruns, 6 failed, 2 dead jobs:

• 2 dead jobs were teuthology/infra issues (Could not get lock /var/lib/dpkg/lock-frontend, SSH connection to smithi038 was lost)
• 1 job failed installing a kernel [Errno 2] No such file or directory: '/var/cache/dnf/CentOS-BaseOS-7c3d8b2bb640713c/packages/kernel-4.18.0-448.el8.x86_64.rpm' unrelated to our tests
• 2 instances of https://tracker.ceph.com/issues/58535
• 3 jobs failed due to changes made to ceph nfs cluster create command in another PR in the run

Overall, should only block merging for PRs related to nfs