snapshot_create_as: Add more test cases

Add reuse external diskonly negative case
Add "--quiesce" without unix channel negative case
Add external memory snapshot compressing cases 
Add test case for bug 872292 …
@will-Do I'm not aware this when the utils_config module in PR status, the set_* functions is for help customize result if you need.

Yes, the set_string works for this as it use quote "" also for given string. I'll update this use the function.