Implemented get_prerelease_data

Implemented get_prerelease_data

Collected all pre-release paths and url making in one place:
application level pre-release API get_prerelease_data. This function
returns all data, required by prerelease applications.

This function can be moved under common/ when there will be more than
one consumers of prerelease data.

Left only read_ and get_ methods in BackendDB as the rest of
functionality is implemented in get_prerelease_data.

Change-Id: I40a311f98d32002cdd10931baa80b75f9b2955de
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>