Fix CentOS 7 OS check

When using awk in CentOS 7, the number is the 4th item instead of the 3rd. To remediate this instead use a regular expression to match on the whole string and ensure a period doesn't precede 6 or 7. However implementing the Perl regex engine with `grep -P` to use a negative lookbehind to make sure we don't match against 6 or 7 if it is preceded by a period.

Does match:
CentOS Linux release **6.6.8**
CentOS Linux release **7.0.1406**
CentOS Linux **6.4**

Doesn't match:
CentOS Linux release 5.7
CentOS Linux 5.6.8