Fail agent_install if agent is already installed

The PE bash installer script doesn't support re-configuring an existing
agent, which is critical to our use case. Therefore, we should fail if
the agent is already installed, since we cannot guarantee correct
configuration in that scenario.

Author: reidmv

tasks/agent_install.sh

@@ -2,6 +2,11 @@
 
 set -e
 
+if [ -x "/opt/puppetlabs/bin/puppet" ]; then
+  echo "ERROR: Puppet agent is already installed. Re-install, re-configuration, or upgrade not supported. Please uninstall the agent before running this task."
+  exit 1
+fi
+
 flags=$(echo $PT_install_flags | sed -e 's/^\["*//' -e 's/"*\]$//' -e 's/", *"/ /g')
 
 curl -k "https://${PT_server}:8140/packages/current/install.bash" | bash -s -- $flags