prefer aws creds set via env

also, set aws access key id from default session, in the event that it 
wasn't set in psiturk's config files but was set in ones that boto 
prefers

psiturk/amt_services.py

@@ -268,6 +268,12 @@ def setup_mturk_connection(self):
             kwargs['aws_secret_access_key'] = aws_secret_access_key
 
         self.mtc = boto3.client('mturk', **kwargs)
+
+        # aws access key might have been set via env var -- fetch it and
+        # set it to the psiturk config for dashboard use
+        self.config.set('AWS Access', 'aws_access_key_id',
+                        boto3.DEFAULT_SESSION.get_credentials().access_key)
+
         return True
 
     def verify_aws_login(self):
@@ -325,7 +331,7 @@ def configure_hit(self, hit_config):
         for qual_id in hit_config['block_qualification_ids']:
             quals.append(dict(QualificationTypeId=qual_id,
                               Comparator='DoesNotExist'))
-        
+
         for advanced_qual in hit_config['advanced_qualifications']:
             quals.append(dict(advanced_qual))
 

psiturk/psiturk_config.py

@@ -71,7 +71,12 @@ def load_config(self):
                 self.set(bc['in_section'], bc['prefer_this'], os.environ.get(env_key))
 
         # prefer environment
-        these_as_they_are = ['PORT', 'DATABASE_URL']  # heroku sets these
+        these_as_they_are = [
+            'PORT',
+            'DATABASE_URL',
+            'AWS_ACCESS_KEY_ID',
+            'AWS_SECRET_ACCESS_KEY'
+            ]  
         for section in self.sections():
             for config_var in self[section]:
                 config_var_upper = config_var.upper()