Allow the parameterisation of a test on an existing variable from the command line

[vkarak]

It's a common pattern if you have a test with a variable that can take multiple values to be able to parametrise the test on that variable. Currently, this cannot be done from the command line and we have to write some boilerplate code. For example,

class MyTest(...):
    x = variable(int, value=0)

class MyTestWithX(MyTest):
    xx = parameter(range(10))

    @run_after('init')
    def set_x(self):
        self.x = self.xx

Ideally, users should be able to do --parameterise=x:1,2,3,4