[BUG]Task pod can‘t write file to fluid_path after setting the accessModes with ReadWriteMany and fuse-opts with rw in dataset ymal

[sallylxl]

What is your environment(Kubernetes version, Fluid version, etc.) Kubernetes v1.18 Fluid 0.5.0

Describe the bug Task pod can‘t write file to fluid_path after setting the accessModes with ReadWriteMany and fuse-opts with rw in dataset ymal

What you expect to happen: Task pod can write file to fluid_path, and also to the dataset path in fluid_path

How to reproduce it

1. create dataset.ymal which sets the accessModes with ReadWriteMany for Dataset and fuse-opts with rw for Alluxioruntime
2. create and run a pod to write file to the fluid_path

Additional Information

[xieydd]

Not support write to cache now? cc @cheyang @TrafalgarZZZ