configure_dmc.yml breaks Cluster Manager config if DMC runs on CM

[igsher]

Task "Configure DMC as a IDXC member" in configure_dmc.yml attempts to configure the node as IDXC SH even if the node is already configured as IDXC Manager

Proposed fix:

- name: Configure DMC as a IDXC member
  include_tasks: configure_idxc_sh.yml
  when:
    - "'clustermanager' not in group_names"

[arcsector]

DMC shouldn't run on the CM as per Splunk's docs, but this is an easy PR to make, feel free to submit it.

[igsher]

DMC shouldn't run on the CM as per Splunk's docs ...

That's incorrect. The docs actually say here and here that under specific circumstances DMC can be colocated on the manager node.