Avoid insufficient EC pipelines in the container pipeline cache

OM cached pipeline may not cache container's all datanode information (for example, some DNs may not report in time), which can be problematic for EC files. If the nodes num is less than the EC data-blocks num, an error will occur when reading data:  

There are insufficient datanodes to read the EC block 

This err will persist until the cache expires (after 6 hours) even if all DNs corresponding to the EC file are completely normal.

So, we should not cache EC pipelines with incomplete data-nodes.