Document usage of CLI variables (CLI guide)

We should add some examples of CLI variables usage into management-cli-guide.

Upstream documentation - https://developer.jboss.org/wiki/CommandLineEnvironmentVariables

For interactive CLI session, 'set' and 'unset' commands are available.

Other ways how to set a CLI variable
- .jbossclirc file JBEAP-3202
- CLI scripting

[pkremens@localhost bin] $ cat test.cli 
set path=/subsystem=logging/

$path:read-resource

[pkremens@localhost bin] $ ./jboss-cli.sh -c --file=test.cli
{
    "outcome" => "success",
    "result" => {
...
}

CLI variables are different to CLI properties (https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/management-cli-guide/#property_substitution)