As a guess...
I would say that the version of the openvas-scanner that you are trying to install; openvas-scanner-3.2-0.2.el5.art.i386 is different from your currently installed client; openvas-client-3.0.1-1.el5.art.i386
So yum is trying to install some dependencies for the scanner, which conflict with the installed dependencies used by the client. This is forcing yum to try to upgrade everything at once to meet requirements - including openvas-client. Hence why yum is telling you about your already installed openvas-client dependencies.
However... The fact that openldap-clients and net-snmp-utils are listed is suggesting a missing distro repo problem... openldap-clients is an un-versioned dependency from your base installs "updates" repo, so it should be resolved from that. This might indicate that you have disabled your rhel-updates repo, or that your subscription has expired.
try the following to see why you can't install the openldap-clients;
# yum info openldap-clients
Loaded plugins: downloadonly, fastestmirror, security
...
Available Packages
Name : openldap-clients
Arch : x86_64
Version : 2.3.43
if that doesn't work try;
yum --enablerepo=base --enablerepo=updates info openldap-clients
and then try
yum --enablerepo=base --enablerepo=updates install openldap-clients net-snmp-utils
Basically, its starting with resolving your distro dependencies, and then see if openvas-scanner still can't update...
Yum does a check on /etc/yum.repos.d/webtatic.repo to see if it has changed, but this appears to not be happening when this file is installed by the second rpm -Uvh command, and yum clean all does not stop this
Webtatic EL6:
-rw-r--r-- 1 root root 805 May 16 22:46 /etc/yum.repos.d/webtatic.repo
Webtatic 5:
-rw-r--r-- 1 root root 834 May 16 22:41 /etc/yum.repos.d/webtatic.repo
Perhaps this is because the Webtatic 5 package was built earlier than 6.
I could add to the build process something that would update the timestamp, however there was never any plan to have Webtatic 5 support CentOS 6, so you may get other unexpected results if you try to use it on that version.
A workaround for now would be to force Yum to spot the file has changed:
touch /etc/yum.repos.d/webtatic.repo
Best Answer
You might be in for a ride, especially if you're missing dependencies. Here is a page where you can view mirrors of the RPM and its dependencies, and search for the dependencies you don't have, in case the following doesn't work.