storage: detect localfs storage

iso-uploader has been designed thinking only at
NFS storage for iso images uploaded using nfs upload backend.
All other storage types have been delegated to ssh upload backend.
Added a check on localfs explaining that for that storage only
ssh upload is supported.

Change-Id: Iff3313ba69500a7dbbdbe4c0972d747a9ae31b99
Bug-Url: https://bugzilla.redhat.com/1065726
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>