bugs in columnstore.cnf

MCOL-5519 added three lines to columnstore.cnf

collation_server = utf8_general_ci

character_set_server = utf8

columnstore_use_import_for_batchinsert = ON

All three are problematic:

• first two change default server character set for the whole server. That is, a user was a working server installation. Installs columnstore — and suddenly the server uses a different default character set. Uninstalls columnstore — columnstore.cnf is removed (on Fedora, for example), and again, suddenly the charset is different. This is a major gotcha, plugin's cnf file should only change plugin's configuration. Fix: remove these lines.
• last line only works when columnstore is installed. But on Debian cnf files are not removed when the plugin is uninstalled. Meaning, after uninstalling columnstore the server will fail to start. Fix: use the loose- prefix.