Bugfix: fatal: destination path '/opt/tempT' already exists and is not an empty direc...

Bugfix: fatal: destination path '/opt/tempT' already exists and is not an empty directory.

JIRA: YARDSTICK-588

ubuntu-server-cloudimg-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: Ia9c35a877207494623055b1b2134f7834fadedf8
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>