[FEATURES] Fluid agent端优化

[cheyang]

What feature you'd like to add:

1.支持alluxioruntime可以创建ramfs https://github.com/fluid-cloudnative/fluid/issues/233 2.支持tiered store对应多个目录，用逗号分割 3.agent可以上报不同盘的存储空间，并且根据此信息做数据集的调度

Why is this feature needed:

[xieydd]

Can you update the motivation of the agent. @cheyang