When running a delete CG operation on a MySQL DB engine we'll get an
error from the DB: "You can't specify target table 'consistencygroups'
for update in FROM clause".
This is caused by a non standard behavior only in MySQL, as SQLite and
PostgreSQL work as expected, on filter `cg_creating_from_src` that
checks that we are not using the CG we are trying to delete to create
another CG at this very moment, CGs that were previously created from
this CG are not considered.
This patch changes the way we perform the filter using a select subquery
as a workaround for MySQL unexpected behavior and updates devref to warn
about this unexpected MySQL behavior.
Change-Id: Ic10de411ffeceb00f1e8525906995bd8b2f49777
Closes-Bug: #1588487
6f63347eef