Detect WAR location properly in Maven

[chyt]

The way we are currently finding the location of the WAR is not robust and only works for the default app name configuration: https://github.com/OpenLiberty/boost/blob/cac4650c89dff1806ddf1721a853a446e4234307/boost-maven/boost-maven-plugin/src/main/java/io/openliberty/boost/maven/liberty/LibertyPackageMojo.java#L181,L186

Instead, I believe we should be evaluating the webAppDirectory parameter from the maven-war-plugin. If the parameter is not set then we should default to that parameter's default value by using project.getBuild().getFinalName().

[scottkurz]

@chyt do you think #387 addresses this at the current level of support? Or do you think we still should make a change?