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
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.

Ensure that the new doctests written as part of this commit are included with
the rest of the doctests.