LVM migrate: Use keywords for the brick instance

In the `migrate_volume` method a new instance is created when the
dest_vg is not equal to the source vg. This new brick instance is
created using positional arguments instead of keywords. However, some of
those arguments are passed in the wrong positions.

This patch uses keywords for the misplaced arguments.

The patch also changes `test_lvm_migrate_volume_proceed` in order to
fully test the happy path and catch things like this.

Closes-bug: #1258128
Change-Id: I75cf91171709554053fd5b52a4aae4e176e8364e
Merge "LVM migrate: Use keywords for the brick instance"