[NIFI-198] Prerequisites is not correctly checked
 Using maven-enforcer-plugin to check the correct maven version.
[NIFI-199] Removed plugin dependencies from main project cause they are
not needed at the moment.
Merge branch 'NIFI-198' of https://github.com/khmarbaise/incubator-nifi into develop
Merge branch 'NIFI-199' of https://github.com/khmarbaise/incubator-nifi into develop
NIFI-198 formatting adjustment, removed comments provided as guides to understand previous patch