Persist automate state data between retries

https://bugzilla.redhat.com/show_bug.cgi?id=1044175

During the execution of an automate method the following 3 methods
can be called to set/get/check for existence of state data.

$evm.set_state_var     This takes a name and value as the 2 arguments
$evm.get_state_var     This takes a name and returns the corresponding value object
$evm.state_var_exist?  This takes a name and checks if the variable exists

The name in all 3 functions above can be a string or a symbol.
If the name is missing nil is returned.