Use download-in-advance mode for zypper migration

This reduces the risk of download problems while inside
of the upgrade process. This Fixes jira#PCT-1170

Author: schaefi

suse_migration_services/units/migrate.py

@@ -79,6 +79,8 @@ def perform(self):
                         '--no-selfupdate',
                         '--auto-agree-with-licenses',
                         '--allow-vendor-change',
+                        '--download',
+                        'in-advance',
                         '--strict-errors-dist-migration',
                         '--replacefiles',
                         '--product',

test/unit/units/migrate_test.py

@@ -141,6 +141,8 @@ def test_main_zypper_migration_plugin(
                 '--no-selfupdate',
                 '--auto-agree-with-licenses',
                 '--allow-vendor-change',
+                '--download',
+                'in-advance',
                 '--strict-errors-dist-migration',
                 '--replacefiles',
                 '--product',
@@ -182,6 +184,8 @@ def test_main_zypper_migration_plugin_verbose(
                 '--no-selfupdate',
                 '--auto-agree-with-licenses',
                 '--allow-vendor-change',
+                '--download',
+                'in-advance',
                 '--strict-errors-dist-migration',
                 '--replacefiles',
                 '--product',
@@ -223,6 +227,8 @@ def test_main_zypper_migration_plugin_solver_case(
                 '--no-selfupdate',
                 '--auto-agree-with-licenses',
                 '--allow-vendor-change',
+                '--download',
+                'in-advance',
                 '--strict-errors-dist-migration',
                 '--replacefiles',
                 '--product',