fix(ShepherdProperties): always load relative to catalina.home

[doinkythederp]

This PR updates ShepherdProperties.loadProperties to load paths relative to the Tomcat install rather than relative to the current working directory. While these are often the same, it is possible that the process will have an arbitrary current working directory, such as when running Tomcat manually or when using the macOS Homebrew service.

Fixes #504.

Changes

• Uses System.getProperty("catalina.home") to get the Tomcat install path. This is more reliable than using the CWD.
• The Java Path API is used for resolution.
• I went ahead and fixed some warnings in the edited function and added documentation to it.
• I've tested this on my Mac but I haven't checked if it works on Linux; I don't have any other computers running Tomcat.

[naknomum]

works on linux with our dev/docker environment.

[naknomum]

looks good and works on my local linux.