Make travis use Makefile when running tests

Do not hard-code `python -m unittest` or `nosetests` statements into the travis
config file. Instead, place those statements in the primary Makefile and make
Travis call the recipes in the Makefile.

Rename recipe "test" to "test-robottelo", which more accurately reflects its
purpose.
Merge pull request #908 from Ichimonji10/travis

Make travis use Makefile when running tests
Simplify ssh command

Leave paramiko handle the execution of the command.
Update a CLI user test to not do any ssh connection during the import
time.
Update requirements, now we can run with the latest paramiko.
Merge pull request #910 from elyezer/improve-ssh-command

Simplify ssh command
Change main configuration file.

Add a new setting to the main config file: `main.server.scheme`. Valid values
for this config option are, for example, http and https.

Create method `get_server_url` in module `robottelo.common.helpers`. This method
constructs the URL of the server being tested by reading `main.server.scheme`,
`main.server.hostname` and `main.server.port`.

Refactor some code which, formerly, constructed the server URL directly from the
config file.

Give method `get_server_url` a doctest. Create a module which runs this doctest
as part of the regular test suite.
Make API docs include new module

New module: tests.robottelo.test_doctests