Implementation and tests to update parameters under settings menu

- As per task #986, boundary testing needs to be done for setting sub-menu. And it includes different parameters with some values. Assuming that we need to set the param values with diff data variations. So based on this assumption, I wrote a:
 -  `Update` fn to edit the selected parm's value. 
 -  `get_saved_value` fn for assertion
 -  some basic tests to test the added functions

Can we raise an Exception subclass? For more info take a look on https://github.com/omaciel/robottelo/blob/master/robottelo/cli/factory.py#L52