Fix centos-8 functional tests
Fix centos-8 functional tests

Centos-8 is the only operating system which excercises the DNF code
path and for functional rather than integrated tests install_method
may be undefined.

Default the install method to 'source'

Change-Id: I628c61e4b846bb380640ed96ee4c1909c88898ff

openstack/openstack-ansible-openstack_hosts,openstack/openstack-ansible-openstack_hosts