check_computer_memory: fix wrong var assignment + update docstring

free were equal to memory_used
used were equal to memory_free

slapos/promise/check_computer_memory/__init__.py

 #!/usr/bin/env python
 
 """
-Check if memory usage is greater than given threshold.
+Check if memory usage is lower than 80% of total memory.
 
 Uses:
 - /proc/meminfo
@@ -29,7 +29,7 @@ def getFreeMemory(database, time, date):
 
     # fetch free and used memory 
     where_query = "time between '%s:00' and '%s:30' " % (time, time)
-    query_result = database.select("system", date, "memory_used, memory_free", where=where_query)
+    query_result = database.select("system", date, "memory_free, memory_used", where=where_query)
     result = zip(*query_result)
     if not result or not result[0][0]: 
       print "couldn't fetch free memory"

slapos/test/promise/test_check_computer_memory.py

@@ -47,7 +47,7 @@ class TestComputerMemory(unittest.TestCase):
     self.conn.close() 
 
   def test_check_memory(self):
-    self.assertEquals({'total': 33705312256.0, 'free': 33139023872.0, 'used': 566288384.0}, 
+    self.assertEquals({'total': 33705312256.0, 'used': 33139023872.0, 'free': 566288384.0},
       getFreeMemory('/tmp', '00:02', '2017-09-15'))
 
   def test_check_memory_with_unavailable_dates(self):