Fix error in instance_get_all_by_filters() use of soft_deleted filter

Change-Id: I7c2fab48944e34765b3fff8ce10bc64a5cd826c8 introduced the
'soft_deleted' filter to the above method to tweek the behavior of the
existing 'deleted' filter. In doing so, an error was introduced that
changed the original behavior of the 'deleted' filter when used by
itself, in how it treated both soft- and hard-deleted instances the
same.

The original merged patch did not include test coverage for the changes
made to the db api. This change fix the error so that the original
behavior of the 'deleted' filter is restored while also adding full
test coverage for the modifications to the db api that has already
merged.

Finally, the support method create_instances_with_args() used by the
new tests was renamed create_instance_with_args() to reflect that
when called, it just creates a single instance.

Resolves bug 1181865.

Change-Id: Ibb82af09d3876904455ca7c93e14e9722ed31d35