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.
Setting `server.scheme` to https is redundant, as https is the default. This fact is stated in a comment directly above the setting. Leaving the option blank reinforces the fact that providing a value for `server.scheme` is optional.

Suggesting "http" and "https" as choices seems rational. I'll do that.