[policy] We need to controll ability to create new network for murano environments via policy

[Kirill Zaitsev]

In many cases cloud admins want murano environments to use a single pre-created network instead. This can be controlled by a policy.

Thus this task should be solved on 2 sides:

1) dashboard (a policy for showing/not showing the 'Create new network' option when creating new environment)

2) api — a policy for editing properties of murano-enging during env creation