REVIEW [NX-556] move configuration files under 'etc' to conform with Karaf layout

This PR updates both the installation and the working directory (sonatype-work) layout so configuration files are expected to be under "etc" instead of "conf". Both places were updated to use "etc" to avoid inconsistency. The impact of this change suggests that it might be worthwhile introducing some basic constants for common directory names (I'll leave that to others to decide and follow up as necessary).

Note that this PR does not perform any migration of files.

https://issues.sonatype.org/browse/NX-556
http://bamboo.s/browse/NX-OSSF192-1 :white_check_mark: