check_env doesn't validate curl is installed

[dhoard-confluent]

check_env should validate curl is installed. A minimal Ubuntu 18.04.2 LTS installation does not install curl by default.

 $ check_running_cp 5.2

This script expects Confluent Platform version 5.2 but the running version is .
To proceed please either: change the examples repo branch to  or update the running Confluent Platform to version 5.2.

If you look at the output of confluent you can see the error ...

$ confluent version
'confluent' requires 'curl'.