ubuntu-server-cloudimg-dpdk-modify.sh: fixes and cleanup

ubuntu-server-cloudimg-dpdk-modify.sh: fixes and cleanup

https://build.opnfv.org/ci/view/fuel/job/yardstick-fuel-virtual-daily-master/1233/console

saw this in an error log, not sure if it is a real error

+ [[ amd64 = \a\r\m\6\4 ]]
+ git clone https://github.com/kdlucas/byte-unixbench.git /opt/tempT
fatal: destination path '/opt/tempT' already exists and is not an empty directory.
+ error_trap
+ local rc=128
+ set +e
+ '[' -z '' ']'
+ exitcode=128

workaround by deletign /opt/tempT always before cloning

also improve ln with relative symlink and forcing filename target

Change-Id: I6715e6dba76b79706d396b616eae980e5ace773b
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>