Wrap command with sh -c for node-execute and run-image step

[ryancurrah]

Lately our team has been onboarding new users to Rundeck and we require them to use this plugin instead of local command.

We have been finding users are having trouble running commands like echo "hello" && echo "bye" or echo "export MYVAR=hello" > vars.env' or cat ~/ansible.cfg until they wrap it in bash -c. This is because of docker not this plugin see docker/compose#2033 for more info.

I propose we wrap commands (If one was specified) in sh -c instead of just passing it normally. This will make it easier for newcomers to docker and this plugin while making the steps more readable.

All major shells (csh, ksh, fish, bash) use the same switch -c to pass commands to them.