zebra/tutorial.rst: use the new `zpm execute` cmd

Previously, it was required to use the `zpm deploy ...  --execute`
command to remotely execute zapps on ZeroCloud. We now have a separate
command for execution in zpm. This change updates the tutorial to use
separate `zpm deploy` and `zpm execute` commands.
zebra/tutorial.rst: add a tip for deploy+execute

This adds a tip to the tutorial for using `zpm deploy ... --execute` as
an alternative to separate commands for deployment and execution.
Merge pull request #19 from larsbutler/zebra-tut-add-execute-cmd

zebra/tutorial.rst: use the new `zpm execute` cmd