ipaserver_setup_kra.py

Since 4.7.1 it is needed to use CustodiaModes.FIRST_MASTER instead of
CustodiaModes.MASTER_PEER for the get_custodia_instance.

This has been fixed already in ipaserver_setup_ca and also
ipaserver_setup_custodia, but was missed in ipaserver_setup_kra.

Fixes: #92 (KRA install fails in tasks: [ipaserver : Install - Setup KRA])