Fix error during custom cluster creation with slow network speeds

[mantis-toboggan-md]

Summary

Fixes #15538

Technical notes summary

This is the line throwing the error

    async saveRoleBindings() {
      await this.value.waitForMgmt();

      if (this.membershipUpdate.save) {
        await this.membershipUpdate.save(this.value.mgmt.id); <------ this.value.mgmt is undefined here
      }
    },

My own local testing and console logs in the issue's screenshot confirm that the wait for management ended because management was found; it didn't time out. The issue appears to be that for a brief window after a provisioning cluster is created, the model's this doesn't include updated status so we can't get the management cluster's id. There is already a workaround for this in waitForMgmt. I added the same workaround to the mgmtId getter

Areas or cases that should be tested

The scenario as described in the issue should be tested. Note for my fellow firefox enjoyers: it seems specific to chrome

Areas which could experience regressions

none

Screenshot/Video

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