Session cleanup for db.security_group_* methods

Remove session parameter from:

 security_group_get
 security_group_get_by_name
 security_group_create
 security_group_update
 security_group_ensure_default

security_group_count_by_project left as is as
it involves quota changes and will be done separately
with other _count_by_project functions.

Convert security_group_destroy to use model_query() instead
of session.query().

blueprint db-session-cleanup

Change-Id: I31e8333f67cf900b6f4aa6c5b967a9a6fb71147e
Merge "Session cleanup for db.security_group_* methods"