Fix remove_config issue for an unexistent file

Unpersisting a unexistent file will cause all the files under config are removed.
For example: The command "unpersist /x/y" will cause all the files under /config
are removed.

Change-Id: I2eea0f9d96e616ad45633243f94ecb84ddd7ae86
Signed-off-by: Wenyi Gao <wenyi@linux.vnet.ibm.com>