Only test CLI for nginx plugin if it is present
Only test CLI for nginx plugin if it is present

  - Fixes 919

jsha/letsencrypt,letsencrypt/letsencrypt,thanatos/lets-encrypt-preview,goofwear/letsencrypt,ghyde/letsencrypt,lbeltrame/letsencrypt,VladimirTyrin/letsencrypt,TheBoegl/letsencrypt,lmcro/letsencrypt,g1franc/lets-encrypt-preview,letsencrypt/letsencrypt,dietsche/letsencrypt,xgin/letsencrypt,g1franc/lets-encrypt-preview,twstrike/le_for_patching,wteiken/letsencrypt,sjerdo/letsencrypt,bsmr-misc-forks/letsencrypt,mrb/letsencrypt,xgin/letsencrypt,bsmr-misc-forks/letsencrypt,stweil/letsencrypt,martindale/letsencrypt,goofwear/letsencrypt,dietsche/letsencrypt,BillKeenan/lets-encrypt-preview,jtl999/certbot,mrb/letsencrypt,mitnk/letsencrypt,martindale/letsencrypt,TheBoegl/letsencrypt,hsduk/lets-encrypt-preview,VladimirTyrin/letsencrypt,hsduk/lets-encrypt-preview,lmcro/letsencrypt,kuba/letsencrypt,DavidGarciaCat/letsencrypt,brentdax/letsencrypt,Sveder/letsencrypt,piru/letsencrypt,stweil/letsencrypt,brentdax/letsencrypt,kuba/letsencrypt,thanatos/lets-encrypt-preview,mitnk/letsencrypt,sjerdo/letsencrypt,jtl999/certbot,Sveder/letsencrypt,lbeltrame/letsencrypt,wteiken/letsencrypt,BillKeenan/lets-encrypt-preview,ghyde/letsencrypt,piru/letsencrypt,twstrike/le_for_patching,jsha/letsencrypt,DavidGarciaCat/letsencrypt