Reduce redundancy & confusion around having two columnstore.cnf files

Serg correctly observed that after merging one of his PRs into develop, we are now deploying both a x-columnstore.cnf and a columnstore.cnf file in both community and enterprise, b/c we don't have a separate branch or repo for each.

For community, the x-columnstore.cnf is unnecessary, and once we're gamma-level, the plugin-maturity option can be removed (b/c gamma is the default there).

His suggestion to avoid having both is to package only columnstore.cnf, and have post-install generate x-columnstore.cnf IF it sees the enterprise-specific .cnf file in /etc/my.cnf.d

Other option is to mod cmake to detect the INSTALL_LAYOUT var, and if enterprise, install x-columnstore.cnf. Else, don't.

Multiple easy options. It should be done before the next release.