py:key_credential_list: add kcl_in_list function

py:key_credential_list: add kcl_in_list function

This compares the key material and DN of a KeyCredentialLinkDn with a
list of others, which is a different sense of equality than the
default (which considers GUIDs and binary equality).

This will be used by samba-tool to check whether a link is in fact a
duplicate even if it seems not to be due to some insignificant field
being non-identical.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>