When s3fs cannot be accessed replaced with osfs

add var XBLOCK_FS_USE_S3_STORAGE ,default to be set True
When aws s3 service cannot be accessed or poor network,webmaster can replace s3fs with osfs

Review ticket:  https://openedx.atlassian.net/browse/OSPR-50
That's what I figured.  @fmyzjs I think the structure you want is something like
```
"DJFS": {
    type: "s3fs"
    # s3fs related settings
    bucket:...
    prefix:...
}
```
or
```
"DJFS": {
    type: "osfs"
    #  osfs related settings
    directory_root:...
    url_root: ...
}
```

@pmitros if the change is structure as I suggest above, do there need to be code changes as well?  If so, can you point to the files where those changes need to go?