The policy rule ``shared_security_group`` allows to create new policy
rules checking if a security group rule belongs or not to the project
default security group.
By default the behaviour has not changed. If an administrator wants
to prevent a non-privileged user from creating or deleting rules in the
default security group, the ``create_security_group_rule`` and
``delete_security_group_rule`` can be overriden. An example is provided
in the unit tests.
Closes-Bug: #2019960
Change-Id: I6c90b61df0e726ef07f177801069baf30c49de67
96223931ca