Clarify workflow and differences between the Installer and Capistrano

[voodoorai2000]

Context

Some users are executing the Installer again to upload new versions of CONSUL to the production servers.

Objectives

Clarify in the documentation that the installer is only meant to me run once.
Future changes to the code in the production server should be done using Capistrano

Notes

CONSUL updates for new Releases should also be done using Capistrano.

However, we still have to decide how to deal with system updates. For the moment, system administrators are expected to do this job, but maybe in the future the Installer can handle upgrading system libraries.

[voodoorai2000]

Closed in favor of consuldemocracy/installer#82