feat: Added possibility to setup NB configuration from file at VPP-Agent start

[fgschwan]

Added possibility to define NB configuration as file and to picked up and applied by VPP-Agent at start.
Additional features:

• file defined as yaml
• ability to set up also 3rd party models (using the same components as are used for agentctl config update --replace, see this issue for understanding more about 3rd party model definition/problems)
• by default active, no additional configuration needed to use (create nb-initial-config.yaml in vpp-agent working directory, i.e. if you installed vpp-agent binary in one of paths from env.variable then nb-initial-config.yaml must be in that directory where you are when you execute vpp-agent binary - if you are still not sure about the location, don't worry, VPP-agent will tell you in debug logging where it expects the file to be by using absolute file path)
• fully configurable (path to configuration yaml file + feature on/off switch)
• usable in projects using VPP-Agent as its base (using the updated code in VPP-Agent plugin setup will allow usage of this feature)
• basic merging capabilities for NB configuration coming from different data sources (NB init file source is just another source among others like etcd and others) - in case of colliding data, other sources have priority to initial NB config file
• e2e tests (start with NB init file, start with Etcd with data, start with both sources having data and merge colliding data - etcd data is used in the end)

[fgschwan]

I ran e2e and integration test for VPP 20.01/20.05/20.09 and it passed for current PR state.

[fgschwan]

I ran e2e and integration test for VPP 20.01/20.05/20.09 and it passed for current PR state.

[fgschwan]

Rebased changes from merged e2e test changes (#1770).
Unit/integration/e2e tests for vpp 20.05 passed ok.