Utils: fix usage of strtime

As oslo_utils.timeutils.strtime() is deprecated in
version '1.6', and will be removed in a future version.

For more informations:
https://docs.openstack.org/oslo.utils/latest/reference/timeutils.html

Change-Id: I1aca257fbe8b08c3478c5da9639835033b19144a
Partial-Bug: #1715325

watcher/common/context.py

@@ -15,8 +15,6 @@ from oslo_log import log as logging
 from oslo_utils import timeutils
 import six
 
-from watcher.common import utils
-
 LOG = logging.getLogger(__name__)
 
 
@@ -102,7 +100,7 @@ class RequestContext(context.RequestContext):
             'domain_name': getattr(self, 'domain_name', None),
             'auth_token_info': getattr(self, 'auth_token_info', None),
             'is_admin': getattr(self, 'is_admin', None),
-            'timestamp': utils.strtime(self.timestamp) if hasattr(
+            'timestamp': self.timestamp.isoformat() if hasattr(
                 self, 'timestamp') else None,
             'request_id': getattr(self, 'request_id', None),
         })