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.

By default, comment out two settings in the config file: `main.server.scheme`
and `main.server.port`. This is in line with what many venerable daemons such as
sshd and vsftpd do. This change has the potential to break code, but the
introduction of method `get_server_url` should mitigate any negative effects.

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. If possible,
make that code use `get_server_url` instead.

Create a unit test for `get_server_url`.
Create some simple tests for path api/v2/hosts

Implement a few tests which check whether it's possible to GET the path, and add
some partially written tests for doing other things like POSTing and messing
with sub-paths of api/v2/hosts.
Create a new wrapper for `requests`.

The wrapper function `request` in module `robottelo.api.client` is intended to
replace the similar wrapper function in `robottelo.api.base`.