avoid risk caused by using count() check user-define-value.

In some scripts,
we use count() to check if the value of self-variable,
needed by special circumstances, has been set or not.
But, it will bring some problem. for example,

 The self-variable declared at the corresponding config file.

 iface_name = "ENTER.BRIDGE.NAME"

 If the user just delete the prompt message and keep it "none".
 the following code will regard 'none' as a valid value without
 any error.

 iface_name = params.get("iface_name")
 if iface_name.count("ENTER"):
     raise error.TestNAError("Please input a existing bridge/ethernet name")

This patch fixes the problem.
Merge pull request #390 from PandaWei/default

avoid risk caused by using count() check user-define-value.