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.
Slap together some prototype tests

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