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`.