Do not require valid migration_interface for controllers

Controllers lacking compute should not be required to provide
valid migration_interface as it is not used there (and prechecks
do not check that either).

Inclusion of libvirt conf section is now conditional on service type.
libvirt conf section has been moved to separate included file to
avoid evaluation of the undefined variable (conditional block did not
prevent it and using 'default' filter may hide future issues).
See https://github.com/ansible/ansible/issues/58835


Additionally this fixes the improper nesting of 'if' blocks for libvirt.

Change-Id: I77af534fbe824cfbe95782ab97838b358c17b928
Closes-Bug: #1835713
Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>