yamllint: Fix missing document start.

Recently added Ubuntu configuration filesi for roles miss document
start marker.

roles/ipabackup/vars/Ubuntu-18.04.yml

 # vars/Ubuntu.yml
+---
 ipaserver_packages: [ "freeipa-server" ]
 ipaserver_packages_dns: [ "freeipa-server-dns" ]
 ipaserver_packages_adtrust: [ "freeipa-server-trust-ad" ]

roles/ipaclient/vars/Ubuntu-18.04.yml

 # vars/Ubuntu-18.04.yml
+---
 ipaclient_packages: [ "freeipa-client" ]
 # Ubuntu Bionic Beaver must use python2 as Python interpreter due
 # to the way python-ipalib package is defined.

roles/ipareplica/vars/Ubuntu-18.04.yml

 # vars/Ubuntu.yml
+---
 ipareplica_packages: [ "freeipa-server" ]
 ipareplica_packages_dns: [ "freeipa-server-dns" ]
 ipareplica_packages_adtrust: [ "freeipa-server-trust-ad" ]

roles/ipaserver/vars/Ubuntu-18.04.yml

 # vars/Ubuntu.yml
+---
 ipaserver_packages: [ "freeipa-server" ]
 ipaserver_packages_dns: [ "freeipa-server-dns" ]
 ipaserver_packages_adtrust: [ "freeipa-server-trust-ad" ]