1. Add 7 API python test scripts for tag immutability;
2. Swagger file has object struct defination issue on imuutability policy rule, because
this policy is in shared with retention rule, therefore, they should be unified to
the same type of rule;
3. Fixed a timeout issue of action keyword, waiting for a long period;
4. Add API test scripts for scan signed image.
Signed-off-by: danfengliu <danfengl@vmware.com>
