Use different repositories for different MaxScale versions

With the way that our MaxScale packaging has worked, all MaxScale releases have gone into a single repository. This means that if a user has MaxScale 1.2.0 installed, and they execute something like the following:

yum update maxscale

This would install the latest release of MaxScale, which is currently 1.4.3. Unfortunately, 1.4.3 is not backward compatible with 1.2.0, so in order to get MaxScale functioning again, the user would have to follow the steps outlined here:

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale/upgrading-maxscale-from-13-to-14/

It seems like we should package MaxScale in such a way that users can't accidentally upgrade to a version that is not backward compatible with their current version.

For example, maybe MaxScale 1.2, 1.3., and 1.4 could have different repositories, similar to how MariaDB 10.0 and 10.1 have different repositories.