adding indentation rule

The yamllinter complains when list are not "properly" indented, ie:

  cct:
    client-classes:
    - pxe_efi_BC
    - pxe_bios

The new indentation rule allows for this syntax, but it has to be consistent per file (https://yamllint.readthedocs.io/en/stable/rules.html#module-yamllint.rules.indentation).