Changed JSON on MutableDict in CapacityLog

Changed JSON type on MutableDict type of
report field in CapacityLog object.
Now doesn't need to rewrite whole dict
in case of partial changes.

Change-Id: I828bfabfa89e9a6c1a8eef55cb5575400868d42f
Partial-Bug: #1482658

nailgun/nailgun/db/sqlalchemy/models/base.py

@@ -23,6 +23,7 @@ from sqlalchemy import DateTime
 from sqlalchemy import Integer
 
 from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.ext.mutable import MutableDict
 from sqlalchemy.orm.base import object_state
 
 from nailgun.db import deadlock_detector as dd
@@ -55,5 +56,5 @@ class CapacityLog(Base):
     __tablename__ = 'capacity_log'
 
     id = Column(Integer, primary_key=True)
-    report = Column(JSON)
+    report = Column(MutableDict.as_mutable(JSON))
     datetime = Column(DateTime, default=lambda: datetime.now())