Storage policy cleaned, storage class explicit loading

Now able to load a storage class explicitely, out of any storage policy context.

This is necessary for the metacd-http to process load-balancing requests with stgclass constraints. In facts, the metacd_http simply wraps the calls to metautils/lib/lb.h API.

Then, the code has been cleaned, simplified and reordered by puprose.

Change-Id: Ibd66594d53666f5ca4d1fab6d4226e1dc17ba95d