storage: avoid use of undefined GLUSTER_CLI variable
    
    Previous commit tried to change configure logic such that the
    GLUSTER_CLI parameter would always be set:
    
      commit 9e97c8c0f0f3921d06bac2b92cd094a41373f748
      Author: Peter Krempa <pkrempa@redhat.com>
      Date:   Mon Jan 9 15:56:12 2017 +0100
    
        storage: gluster: Remove build-time dependency on the 'gluster' cli tool
    
    This missed the fact that the AC_PATH_PROG call was itself inside an 'if'
    conditional that would not be called in with_storage_gluster was false. As
    a result, GLUSTER_CLI was still conditionally defined.
    
    Just kill the GLUSTER_CLI parameter and AC_PATH_PROG call entirely and pass a
    bare "gluster" string to virFindFileInPath instead.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>