storage: avoid use of undefined GLUSTER_CLI variable

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>