Do not load Task.cache column by default
Do not load Task.cache column by default

For 100 nodes, cache column may have 70MB. For sqlalchemy it takes almost 10s
to load it. This column wil be loaded only when accessed.

Closes-Bug: 1381757
blueprint 100-nodes-support

Change-Id: I220faf3b49244f3eda8bde26ede85f149eb2caa2

zhaochao/fuel-web,huntxu/fuel-web,SmartInfrastructures/fuel-web-dev,huntxu/fuel-web,zhaochao/fuel-web,eayunstack/fuel-web,prmtl/fuel-web,nebril/fuel-web,prmtl/fuel-web,nebril/fuel-web,eayunstack/fuel-web,prmtl/fuel-web,stackforge/fuel-web,SmartInfrastructures/fuel-web-dev,SmartInfrastructures/fuel-web-dev,zhaochao/fuel-web,SmartInfrastructures/fuel-web-dev,zhaochao/fuel-web,huntxu/fuel-web,eayunstack/fuel-web,prmtl/fuel-web,eayunstack/fuel-web,nebril/fuel-web,prmtl/fuel-web,stackforge/fuel-web,nebril/fuel-web,zhaochao/fuel-web,nebril/fuel-web,stackforge/fuel-web,eayunstack/fuel-web,huntxu/fuel-web,huntxu/fuel-web,SmartInfrastructures/fuel-web-dev