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[1]

I suggest to treat the 7.3 and future new versions like the last version certified, 7.2 in this case.

Thanks

Dario

[1] https://www.howtoforge.com/community/threads/amavis-start-but-not-work-via-misconfigurated-sock-option-c7-2-ispc3-1.75149/