Fixing CLI User tests.

* The User command in foreman does not allow for getting a record's
  information (using the ```info``` argument) using ```--login```, only
  ````---id````. So I have fixed user tests so that ```login``` is used
  for assertions and updates.
* I noticed that data from test modules were getting overriden by the
  default data sets included in
  ```robottelo/cli/metatest/template_methods.py```, so I fixed it as
  well.
* Updated CLI base ```exists``` method to return a
  ```SSHCommandResult``` object instead a single record from Foreman.
* Updated all CLI tests that use ```exists```.
Merge pull request #302 from omaciel/fix-cli-user-tests

Fixing CLI User tests.