[SPARK-2777][MLLIB] change ALS factors storage level to MEMORY_AND_DISK

Now the factors are persisted in memory only. If they get kicked off by later jobs, we might have to start the computation from very beginning. A better solution is changing the storage level to `MEMORY_AND_DISK`.

srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #1700 from mengxr/als-level and squashes the following commits:

c103d76 [Xiangrui Meng] change ALS factors storage level to MEMORY_AND_DISK