generate-ceph-tarball: define git_branch variable

The git_ref variable was unused.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
hardcode v3.10.0 git tag

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
build against kernel 3.10.0-123.9.3.el7

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>