Add warning that --platform auto is not supported with ec2 subcommand.

This is not currently supported because a script must be executed on the
remote node to determine its platform and version *before* bootstrapping
begins. Since the knife-ec2 plugin creates a new server and bootstraps
it as one operation, this makes platform detection via script
nontrivial.
Fix up --log-level flag.
[Breaking] Add --webui-enable flag & default to disabling webui.

This change is in line with upstream defaults for future Chef Server
package releases. For more details, please see:

http://lists.opscode.com/sympa/arc/chef-dev/2013-02/msg00023.html