Disable jsonschema validation by default (Qiskit#688)

Currently the aer backends are running jsonschema validation on the Qobj
objects passed in via terra by default. This is unecessary overhead as
terra is programatically creating these objects and they're extremely
unlikely to be structurally invalid (which is all jsonschema will
catch). Even, if it was this will be caught later when something goes to
use it and the error message from that is more likely to be useful since
the way the json schemas are constructed precludes useful error message.
This commit changes the default value of this to be opt-in on the run()
method to avoid this unecessary overhead.

Co-authored-by: Christopher J. Wood <cjwood@us.ibm.com>

qiskit/providers/aer/backends/aerbackend.py

@@ -83,7 +83,7 @@ def __init__(self, controller, configuration, provider=None):
         self._controller = controller
 
     # pylint: disable=arguments-differ
-    def run(self, qobj, backend_options=None, noise_model=None, validate=True):
+    def run(self, qobj, backend_options=None, noise_model=None, validate=False):
         """Run a qobj on the backend.
 
         Args:

qiskit/providers/aer/releasenotes/notes/validation-opt-in-e43a002f160ddd11.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    For Aer's backend the jsonschema validation of input qobj objects from
+    terra is now opt-in instead of being enabled by default. If you want
+    to enable jsonschema validation of qobj set the ``validate`` kwarg on
+    the :meth:`qiskit.providers.aer.QasmSimualtor.run` method for the backend
+    object to ``True``.