unify functest_yaml obtain process

unify functest_yaml obtain process

functest_yaml is needed in almost all of the testcases, the obtain process is the same:
with open(os.environ["CONFIG_FUNCTEST_YAML"]) as f:
functest_yaml = yaml.safe_load(f)
f.close()
abstract a method to unify the process, and provide the interface

JIRA: FUNCTEST-447

Change-Id: I96a9a9962d7b466f10bbd3b5ab2495957524e22a
Signed-off-by: SerenaFeng <feng.xiaowei@zte.com.cn>