Replace all explicit version strings with the ENV variable NIFI_VERSION
Replace all explicit version strings with the ENV variable NIFI_VERSION

Since this same Dockerfile will mostly be used for 0.3.0 when it comes out, change all explicit 0.2.1 strings to use the previously set ENV variable NIFI_VERSION

GDATAAdvancedAnalytics/docker-nifi,GDATAAdvancedAnalytics/docker-nifi,mkobit/docker-nifi,mkobit/docker-nifi