make scylla_util OS detection robust against empty lines
make scylla_util OS detection robust against empty lines

Newer versions of RHEL ship the os-release file with newlines in the
end, which our script was not prepared to handle. As such, scylla_setup
would fail.

This patch makes our OS detection robust against that.

Fixes #4473

Branches: master, branch-3.1
Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20190502152224.31307-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla