[EC2SpotManager] EC2 internal error should be temporary error

Similar to #585, we get internal errors on EC2 that we have no control over. We should just retry later.

start_instances: unhandled error: Traceback (most recent call last):
File "/srv/fmo/FuzzManager/server/ec2spotmanager/CloudProvider/CloudProvider.py", line 54, in wrapper
return wrapped(*args, **kwds)
File "/srv/fmo/FuzzManager/server/ec2spotmanager/CloudProvider/EC2SpotCloudProvider.py", line 81, in start_instances
timeout=10 * 60):
File "/srv/fmo/laniakea/laniakea/core/providers/ec2/manager.py", line 184, in create_spot_requests
requests = self.ec2.request_spot_instances(price, valid_until=valid_until, **self.images[instance_type])
File "/usr/local/lib/python2.7/dist-packages/boto/ec2/connection.py", line 1647, in request_spot_instances
verb='POST')
File "/usr/local/lib/python2.7/dist-packages/boto/connection.py", line 1170, in get_list
response = self.make_request(action, params, path, verb)
File "/usr/local/lib/python2.7/dist-packages/boto/connection.py", line 1116, in make_request
return self._mexe(http_request)
File "/usr/local/lib/python2.7/dist-packages/boto/connection.py", line 1028, in _mexe
raise BotoServerError(response.status, response.reason, body)
BotoServerError: BotoServerError: 500 Internal Server Error
<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InternalError</Code><Message>An internal error has occurred</Message></Error></Errors><RequestID>3b811d6d-d6cd-42b7-a37b-32368ddd487b</RequestID></Response>

Start in server/ec2spotmanager/CloudProvider/EC2SpotCloudProvider.py at start_instances, then inspect laniakea/core/providers/ec2/manager.py around create_spot_requests and the CloudProvider wrapper. Compare the related #585 issue and trace how the shown BotoServerError is handled. Done means EC2 InternalError responses are treated as temporary and retried later rather than reported as unhandled.