mgr/dashboard: Ensure E2E tests can be run independently

mgr/dashboard: Ensure E2E tests can be run independently

Introduces the PageHelper.

- Reads class attributes from Helpers to reduce boilerplace code for
  navigation.
- The PageHelper is supposed to be the new class for code that's reused
  across all Helpers for page related code.
- The Helper class is by now meant to be used for non-page specific
  helper code, like used in the `checkConsole` method.
- The Helper class will act as central registry to enable all other
  tests to use helper functions of all derived PageHelper tests.

  Example:

  `Helper.pools.create('foobar', ...);`

Fixes: http://tracker.ceph.com/issues/40397
Signed-off-by: Patrick Nawracay <pnawracay@suse.com>