Allow setting reseller_prefix for keystone filter

When using keystone, it's possible to modify the behaviour to
use a different prefix instead of the default AUTH_.

This patch allows you to be able to set that option when
declaring your class definition.

Change-Id: I11ff23d118e0f3d1792b43e1dc7a2c82c6826427
Merge "Allow setting reseller_prefix for keystone filter"