Add a function to check the cacert file

Add a function to check the cacert file

1. Currently it won't pass the cacert file to other containers if the
   OS_AUTH_URL is http not https.
2. However, even though the OS_AUTH_URL is http, it still needs cacert file
   somewhere for Functest and Rally.
3. This problem is found on Fuel Euphrates.
4. Add a function to check whether the cacert file exists.
5. If exists, pass it to testing project containers.

JIRA: DOVETAIL-616

Change-Id: Ied7bcc72e8f1c738bbce32c18096ca13641d3cd7
Signed-off-by: xudan <xudan16@huawei.com>