Fix lxd_container module fails if certificate already in trust store

When the client certificate is already stored, lxd returns a JSON error with message "Certificate already in trust store". This "error" will occur on every task run after the initial run. The cert should be in the trust store after the first run and this error message should really only be viewed as informational as it does not indicate a real problem.

Fixes:
ansible/ansible-modules-extras#2750

lib/ansible/module_utils/lxd.py

@@ -121,6 +121,8 @@ class LXDClient(object):
             if resp_type == 'error':
                 if ok_error_codes is not None and resp_json['error_code'] in ok_error_codes:
                     return resp_json
+                if resp_json['error'] == "Certificate already in trust store":
+                    return resp_json
                 self._raise_err_from_json(resp_json)
             return resp_json
         except socket.error as e: