write test results to a local file

write test results to a local file

Write test result to a file or push it to DB depends
on the value format of test_db_url which is defined in
config_functest.yaml. Meanwhile, test_db_url can be set
by os envrion value "RESULT_STORE".

If test_db_url is a url, e.g. http:// or https://, then
push result to DB. If test_db_url is a file location,
e.g. file:///, then write results to a file with json data.
One result record, one line.

JIRA: FUNCTEST-657

Change-Id: I579087cd2c24d61a79142b5d67003fb486b6c723
Signed-off-by: wu.zhihui <wu.zhihui1@zte.com.cn>