Set KRB5CCNAME so httpd s4u2proxy can with with newer krb5-server

The DIR ccache format is now the default in krb5-server 1.11.2-4
but /run/user/<uid> isn't created for Apache by anything so it
has no ccache (and it doesn't have SELinux permissions to write here
either).

Use KRB5CCNAME to set a file path instead in /etc/sysconfig/httpd.

https://fedorahosted.org/freeipa/ticket/3607