ceph-setup: wipe workspace

ceph-setup: wipe workspace

The ceph-setup task occasionally gets weird errors that can only be
fixed by wiping the workspace.

The latest example:

    tardir=ceph-0.94.1.3 && ${TAR-tar} chof - "$tardir" | GZIP=--best gzip -c >ceph-0.94.1.3.tar.gz
    tar: ceph-0.94.1.3/src/gmock/gtest/xcode/Samples/FrameworkSample/WidgetFramework.xcodeproj/project.pbxproj: file name is too long (max 99); not dumped
    tar: ceph-0.94.1.3/src/test/ubuntu-12.04/debian: File removed before we read it
    tar: ceph-0.94.1.3/src/test/ubuntu-14.04/debian: File removed before we read it
    tar: ceph-0.94.1.3/src/test/debian-jessie/debian: File removed before we read it
    tar: Exiting with failure status due to previous errors

... btw, <3 the comment in this job's description, "The clear workspace
before checkout box for the git plugin is used."

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