add support for passing a sys_config to common test

[tsloughter]

Follows the same pattern as shell to set env variables for applications from a config file but in this case for the common test provider.

Examples:

$ rebar3 ct --sys_config=config/sys.config

In rebar.config:

{ct_opts, [{sys_config, ["config/sys.config"]]]}

[ferd]

Would this fit better inside rebar_config?

[tsloughter]

Na, that is really mostly for the rebar.config functions.