flow logs for non-ACCEPT events only

[pznamensky]

What would you like to be added:
Currently, network policy events can be written to network-policy-agent.log.
It's a very convenient way to debug network policy problems.
However, when lots of connections are being established, the status of all of them has to be written to disk.
This creates unnecessary and sometimes high pressure on the CPU & disk.
In our case, we use these logs for easier application of NetworkPolicies and I thought that it might make sense to be able to configure what type of events should be written to the log.
Specifically, I think for this case either only events with verdict: DENY or events without verdict: ACCEPT are needed.

Why is this needed:
To not burn CPU cycles.