Default network admin password change is not persistent across controller restart

Default network admin password change is not persistent across controller restart

ISSUE: Change default network admin (NA) user password, save config, restart controller.
       To be able to login as default NA user you still have to type the default password.
       This happens because when parsing startup configuration user config addition are replayed
       through public API which has a check to block addiiton/removal of default NA user.

CHANGES:
- Perform checks in a middle layer function and have a internal level function which just update the db
- Call the internal function during startup config replay
- Added missing checks on presence/conflict of user configs when removing/adding a user config object

Change-Id: I38c98bb946b42215cec46d86d3b1a9885c12795b
Signed-off-by: Alessandro Boch <aboch@cisco.com>