storage conf: Add support for key-value pair options for pools

This allows the end-user to pass down options to the storage pool
backend.

For example NFS could get mount options or Ceph librados options
passed down.
storage conf: Support setting RADOS options in RBD storage backend

This way users can manually set options in librados which might suite
their needs better.
storage conf: Support mount options for NetFS pools

This way users can provide mount options for for example NFS
storage pools.