codegen, sdk: Use relative paths

Currently the SDK assumes that resources are located in the /api URL,
but this might not be true, depending on the configuration of the
server. This patch changes the SDK to use relative paths for all
resources, so it will work with servers configured with any entry point
URL.

Change-Id: I0e39510541e34349fce5a8ad4a643cf65784bccc
Bug-Url: https://bugzilla.redhat.com/1038952
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>