Improve selenium test stability & speed. All tests are repaired.

The main part of my idea is to make tests more stable and faster. I'm planning to do it in a few steps.

Every step should not destroy tests. And also each step is an independent pull-request.(They will be presented as a sub-tasks).

Step 1. Move all selenium tests into new module - "zeppelin-webtests".
Step 2. Update travis configuration ".travis.yml" to run test from this module.
Step 3. Remove unnecessary dependency from module "zeppelin-server". Add new framework etc. to "zeppelin-webtests" pom file.
Step 4. Too large, but its easy to check (test on CI should be green). Rewrite all selenium tests (do it more stable and faster). Do code more cleaner and easily to read. (It’s already done locally). Result is awesome.