Fix KVS create_grant to not raise NotFound if no user/group

The KVS assigment backend would fail with NotFound if the user
or group didn't exist when calling create_grant while the SQL
assignment backend did not fail.

It's unreasonable to check if the user or group exists when
creating a grant, so the KVS assignment backend is changed to
not require the user or group to exist.

Closes-Bug: #1239476
Change-Id: I2d3b2fd82e7d4d33ac7158968d72cbfbe37528dc
Merge "Fix KVS create_grant to not raise NotFound if no user/group"