As a Developer I want that Ansible scripts support deployment of Icelandic Security Server so that it's automatized

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In task XRDDEV-916 the Icelandic metapackage (xroad-securityserver-is) for the Security Server was integrated. Related to this we need to add support to the Ansible scripts for deploying the Security Server with the Icelandic metapackage. That requires adding a new variant package (xroad_varpkg_is: xroad-securityserver-is):
https://github.com/nordic-institute/X-Road/blob/develop/ansible/roles/xroad-ss/defaults/main.yml

The JIRA ticket this issue was created from can be found here: https://nordic-institute.atlassian.net/browse/XRDDEV-924

Acceptance criteria:

• Ansible scripts ansible/xroad_dev.yml and ansible/xroad_init.yml can be used to deploy Icelandic Security Server
  • https://github.com/nordic-institute/X-Road/blob/develop/ansible/xroad_dev.yml
  • https://github.com/nordic-institute/X-Road/blob/develop/ansible/xroad_init.yml
• The deployed Security Server uses the Icelandic metapackage (xroad-securityserver-is)
• The deployed Security Server starts correctly
• Documentation is updated:
  • https://github.com/nordic-institute/X-Road/tree/develop/ansible#variant