Make sure that the nexus_root directory exist

if the nexus_root directory does not exist (e.g. $nexus_root = '/srv/nexus') then the module fails.

if the nexus_root directory is /srv then nexus logs the exceptions below because it cannot write to the ${user.home} (/srv) that is owned by root. (see this: https://issues.sonatype.org/browse/NEXUS-3671 )

so to avoid that I added logic to make sure the module has this new functionality while at the same time keep backwards compatibility.