fix(install_scylla_bench): reinstall scylla-bench if it exists
fix(install_scylla_bench): reinstall scylla-bench if it exists

Currently we are still using very old scylla-bench for testing, so
scylla-bench failed in tls jobs. Because tls support of scylla-bench was
added recently.

When we prepare the new loader image, scylla-bench won't be updated by
`go get github.com/scylladb/scylla-bench'.

[centos@longevity-tls-1tb-7d-4-4-loader-node-223f91b5-1 ~]$ ls -lh go/bin/
total 6.3M
-rwxrwxr-x. 1 centos centos 6.3M Jan 24  2019 scylla-bench
[centos@longevity-tls-1tb-7d-4-4-loader-node-223f91b5-1 ~]$ md5sum go/bin/scylla-bench
5cbd2c3e9faff3feab171f89d87d1da8  go/bin/scylla-bench

This patch added a `-u' option to go cmdline, it will upgrade existing
scylla-bench to latest.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests