Refactor the code

Refactor the code

Multiple code refactorings to make the code
simpler and increase the unit testing coverage.

* Golint findings are fixed.
* Xapp query function name changed from status to
  config.

The functionality is slightly changed so that
  - Vespa is started only after the xapp notification
    subscription is successful and the vesmgr has received
    the current xapp configuration from the xapp manager.
  - The vesmgr goes to the main loop after that.

Change-Id: Ie2675c0543d4e4ce0a60b92a6c06a79b9e2cb2cd
Signed-off-by: Roni Riska <roni.riska@nokia.com>