Allowing operator to configure a permission for image file in fs store

In this way the user other service used, e.g. Nova, who consumes the
image could be the exclusive member of the group that owns the files
created.

Closes-bug: 1264302
Related-Id: I4d543d205b0805fe00dcab1b0872c0a5e0f97a5f
Change-Id: Iec8396f92ed11531dccb82957da2455ca333430a
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>