dist/redhat: don't try to adduser when user is already exists

Currently we get "failed adding user 'scylla'" on .rpm installation when user is already exists, we can skip it to prevent error.

Fixes #1958

Signed-off-by: Takuya ASADA <syuu@scylladb.com>
Message-Id: <1482550075-27939-1-git-send-email-syuu@scylladb.com>
(cherry picked from commit f3e45bc)

dist/redhat/scylla.spec.in

@@ -110,8 +110,8 @@ cp -r scylla-housekeeping $RPM_BUILD_ROOT%{_prefix}/lib/scylla/scylla-housekeepi
 cp -P dist/common/sbin/*  $RPM_BUILD_ROOT%{_sbindir}/
 
 %pre server
-/usr/sbin/groupadd scylla 2> /dev/null || :
-/usr/sbin/useradd -g scylla -s /sbin/nologin -r -d %{_sharedstatedir}/scylla scylla 2> /dev/null || :
+getent group scylla || /usr/sbin/groupadd scylla 2> /dev/null || :
+getent passwd scylla || /usr/sbin/useradd -g scylla -s /sbin/nologin -r -d %{_sharedstatedir}/scylla scylla 2> /dev/null || :
 
 %post server
 # Upgrade coredump settings