Truncate test name to maximum allowed length of 256 chars.

`reportportal_client.errors.ResponseError: 4001: Incorrect Request. [Field 'name' should have size from '1' to '256'.]`

See https://reportportal.slack.com/archives/C2GTJTTNH/p1514306996000087

Author: s0undt3ch

pytest_reportportal/service.py

@@ -161,7 +161,15 @@ def post_log(self, message, loglevel='INFO', attachment=None):
 
     @staticmethod
     def _get_full_name(test_item):
-        return test_item.nodeid
+        fullname = test_item.nodeid
+        if len(fullname) > 256:
+            fullname = fullname[:256]
+            test_item.warn(
+                'C1',
+                'Test node ID was truncated to "{}" because of name size '
+                'constrains on reportportal'.format(fullname)
+            )
+        return fullname
 
     @staticmethod
     def _get_description(test_item):