Add option to disable flow logging for network instance

[milan-zededa]

Flow-logging is an EVE feature that allows to collect information
about every application network flow. This includes attributes like
src/dst IP, src/dst port, ACL rule applied, packet/byte counters, etc.

However, this feature is always enabled for every network instance and
can produce lot of data which are then uploaded to the cloud.
We have seen cases where this amounted to hundreds of GB each week,
which was a burden for the controller's database.

Another drawback of flow-logging is that the iptables rules that EVE
installs for network instances are considerably more complicated because
of this feature and thus introduce additional packet processing overhead.

This API change introduced a new boolean option to disable flow logging
for a given Network Instance.
It is recommended that the controller disables flow logging unless it is
explicitly enabled by the user.

[eriknordmark]

This being an "enable" means that the default will be off since the controller(s) would not set it initially. I wonder if it is better making it a "disable" knob and then we can add a properly documented and notified default change to disable it in the controller.

[naiming-zededa]

Or we can offer a configitem for this, if user do want this, they can explicitly enable this per device

[milan-zededa]

OK, changed to disable_flowlog.
Originally, I intended to make this a global configitem, but Daniel asked if this could be a NI option because configitems are not UX friendly.

[zed-rishabh]

I don't see any difference between having an enableFlowlog variable set to False by default and a disableFlowlog variable set to True. Both approaches achieve the same result: disabling flow logs by default. The key point is that the default state of flow logs is determined by the initial value of the variable. Whether we use enableFlowlog = False or disableFlowlog = True, the outcome remains the same.
@eriknordmark Or do you mean that flow logs will be enabled by default from the controller, and at a later point, we'll disable them by default after properly communicating this change to the users?

[eriknordmark]

LGTM