Use custom TLS certificate if provided

Since `urllib3` uses `certifi` as the certificate provider [1], if the
users enable TLS with a custom certificate, the `Service.do_get` method
will not be able to verify the custom certificate. According to the
documentation [1], we need to add `cert_reqs` and `ca_certs` to the
`urllib3.PoolManager` when a custom certificate is used.

[1] https://urllib3.readthedocs.io/en/latest/advanced-usage.html#custom-tls-certificates

Story: #2011168
Task: #50514
Change-Id: I75857cf0d0d37254180aa3ae2305e16610c9fab4

config_tempest/services/base.py

@@ -59,6 +59,10 @@ class Service(object):
             if self.disable_ssl_validation:
                 urllib3.disable_warnings()
                 http = urllib3.PoolManager(cert_reqs='CERT_NONE')
+            elif self.ca_certs is not None:
+                http = urllib3.PoolManager(
+                    cert_reqs='REQUIRED', ca_certs=self.ca_certs
+                )
             else:
                 http = urllib3.PoolManager()
             r = http.request('GET', url, headers=self.headers)