mgr/dashboard: Add decorator to skip parameter encoding

mgr/dashboard: Add decorator to skip parameter encoding

By enconding all parameters of api services we were also encoding parameters
that were being sent in the body of the request.
Those parameters don't need to be enconded and the server never decodes them.

With this new decorator you can specify if you don't want a parameter to be
enconded.

This is a regression introduced in f21d0da5a3e19e3f9bbde084e71b4a09f8dcb0a1.

Fixes: http://tracker.ceph.com/issues/26856
Signed-off-by: Tiago Melo <tmelo@suse.com>