Minor changes to make certificates test cases use HTTPRequestV3

The existing certificates test in v3 API version test correctly
but it use HTTPRequest keyword inside its test cases and has v2
keyword. Though it will not affect the test, change to HTTPRequestV3
will be helpful in the future.

Change-Id: I0992152e56a3f5b5972e48cb6cda16ef39cacdd2
Merge "Minor changes to make certificates test cases use HTTPRequestV3"