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
Fix import order in log_handler

Change-Id: I206adc29163a6e36501bf1d157f50ca2d0867507
Remove unused variables in the xenapi.vmops module

The local variables "current_chain" and "ctx" are assigned to but
never used.

Change-Id: I25bacd728bc5e00d768656c0b2aa657a2effda73
Merge "Minor changes to make certificates test cases use HTTPRequestV3"
Merge "Remove unused variables in the xenapi.vmops module"
Merge "Fix import order in log_handler"