CORE-54 Fixed order of Mojo system properties processing.

Corrected the relative position of system properties processing and config initialization in the Maven plugin so system properties in the POM configuration will override defaults again.

Recycled [CORE-54](https://liquibase.jira.com/browse/CORE-54) since technically system properties weren't supported in 3.2.x, and the issue seemed to be in need of cleanup.