cmd/scollector: only run systemd collector if systemctl exists
cmd/scollector: only run systemd collector if systemctl exists

fixes #1623

leapar/bosun,Skyscanner/bosun,leapar/bosun,seekingalpha/bosun,bookingcom/bosun,CheRuisiBesares/bosun,simnv/bosun,mhenderson-so/bosun,alienth/bosun,mhenderson-so/bosun,Skyscanner/bosun,TrentScholl/bosun,seekingalpha/bosun,CheRuisiBesares/bosun,bosun-monitor/bosun,bookingcom/bosun,bookingcom/bosun,alienth/bosun,augporto/bosun,csmith-palantir/bosun,TrentScholl/bosun,mhenderson-so/bosun,noblehng/bosun,TrentScholl/bosun,CheRuisiBesares/bosun,augporto/bosun,augporto/bosun,Skyscanner/bosun,noblehng/bosun,Skyscanner/bosun,alienth/bosun,noblehng/bosun,bookingcom/bosun,GROpenSourceDev/bosun,augporto/bosun,bosun-monitor/bosun,simnv/bosun,leapar/bosun,GROpenSourceDev/bosun,csmith-palantir/bosun,CheRuisiBesares/bosun,bosun-monitor/bosun,Skyscanner/bosun,bookingcom/bosun,simnv/bosun,melifaro/bosun,TrentScholl/bosun,leapar/bosun,csmith-palantir/bosun,GROpenSourceDev/bosun,GROpenSourceDev/bosun,simnv/bosun,leapar/bosun,mhenderson-so/bosun,CheRuisiBesares/bosun,seekingalpha/bosun,mhenderson-so/bosun,GROpenSourceDev/bosun,mhenderson-so/bosun,csmith-palantir/bosun,csmith-palantir/bosun,alienth/bosun,alienth/bosun,csmith-palantir/bosun,alienth/bosun,melifaro/bosun,bosun-monitor/bosun,noblehng/bosun,bosun-monitor/bosun,CheRuisiBesares/bosun,Skyscanner/bosun,leapar/bosun,simnv/bosun,seekingalpha/bosun,seekingalpha/bosun,augporto/bosun,melifaro/bosun,TrentScholl/bosun,melifaro/bosun,GROpenSourceDev/bosun,simnv/bosun,bookingcom/bosun,noblehng/bosun,augporto/bosun,noblehng/bosun,TrentScholl/bosun,seekingalpha/bosun,bosun-monitor/bosun,melifaro/bosun,melifaro/bosun