Clean up attach/detach tests.

Clean up attach/detach tests.

The code tested by test_preattach_status_volume was moved, and the
test no longer tested anything relevant (i.e., instead of checking
that the volume is 'attached', the test was changed to test for
'available' since the attach code was moved). This patch, therefore,
deletes that test.

Further, test_run_attach_detach_volume called database functions for
attaching and detaching. This patch changes those calls to call
higher-level functions which in turn call the database functions. This
gives better code coverage and gives the tests more meaning.

Change-Id: If688fe6a14940148b5792256c8507803e8cde901