Fix security-manager-policy-reload erasing existing entries

Fix security-manager-policy-reload erasing existing entries

The policy reload script, while reloading user type buckets, used to
unintentionally erase existing entries mapping users to user types.
This was caused by the way in which user type buckets were reloaded:
by removing the bucket and recreating it with intended contents.
Erasing the bucket is wrong - it also erases all links to it.
Changing the reload mechanism to clean the bucket instead.

Change-Id: I6279b2f75d1b7136679edf228d89eb2b001bd76b
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>