Enable support to Centos 7.3 for ispconfig setup and update
From some week, if admins run "yum update" on a Centos 7.2, the system become Centos 7.3. This is not avoidable.
In this case, the ISPC setup do not work property because the procedure treat the new 7.3 version like a old 7.0 Centos version and some configuration files like amavisd.conf or dovecot.conf are not property configured
I suggest to treat the 7.3 and future new versions like the last version certified, 7.2 in this case.