restapi: Remove trailing slashes from RSDL prefix

The RSDL document is loaded lazily, as part of the first request to the
RESTAPI. The prefix used to build the "href" attributes is extracted
from the path of this first request, and depending on that request it
may or may not contain a trailing slash. To make sure that the RSDL has
always the same content we need to remove any trailing slashes from the
prefix before using it.

Change-Id: Ieac656924ca999ddc9abc502e3fc6c23537e4769
Bug-Url: https://bugzilla.redhat.com/1102772
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>