Adds test for Sec group default rule Nova V2 API

This patch adds the test for below 'os-security-group-default-rules'
Nova V2 APIs-
- Create Security Group default rule
- Delete Security Group default rule
- List Security Group default rule
- GET Security Group default rule

This patch also adds the new json client.

Related-Bug: 1311500

Change-Id: Icf52a64b5719587cd02b906f3a0f8d17886241aa
Fix javelin2's skip logic in checks

The create logic always requires the top level section in the yaml file,
but it can be empty.

Change-Id: Ia06bba8877be1cf4e66dad21eaeb2a323bf9f9b5
Merge "Adds test for Sec group default rule Nova V2 API"
Merge "Fix javelin2's skip logic in checks"