Add Karaf 4 distribution, fix initial config

Add Karaf 4 distribution, fix initial config

- Add Karaf 4 distribution for testing to distribution/karaf4.
  This distribution will replace current distribution/karaf
  after support for Karaf 3 will be removed.
- Fix using Karaf 3 shell commands dependencies, and use
  correct ones based on Karaf version specified in distribution
- Fix copying of initial configuration file (wrong path of config
  .xml, was `src/main/features/features.xml` but corrected it to
  `src/main/feature/feature.xml`)

Resolves: bug 8693

Change-Id: I9d65f9a369e1f7750724a432e37cef944f9dcfb4
Signed-off-by: Tomas Slusny <tomas.slusny@pantheon.tech>