Warn when attempting to pipe commands

The `execute` helper does not support piping commands due to the
complexity of supporting this in both MRI and JRuby implementations.

In order to prevent misuse, fail loudly to the user if they try to use
the pipe character as one of the arguments in their command.

Change-Id: Id87e3173a6cb4936c2f06733c688201c1a31dada
Reviewed-on: http://gerrit.causes.com/42170
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>