Clean up session in db.sqla.api.network_* and sec_groups_* methods

Remove session parameter from network_get():
Create _network_get() that contains same logic that old network_get(),
for internal usage, and new network_get() doesn't contains session
parameter.

Remove session parameter from:
security_group_default_rule_get()
and
security_group_default_rule_list()

They are never called with session parameter.

blueprint db-session-cleanup

Change-Id: If09d13112a35ca40dd288705cfb727d3c7a74774
Merge "Clean up session in db.sqla.api.network_* and sec_groups_* methods"