Install python3-pip as prerequisite (fixes #498)

docs/installation_instructions.md

@@ -33,7 +33,9 @@ guide][https://openappstack.net/contact.html).
   * At least 25GB of disk space for installation, plus more for application
     data. We recommend starting with 30GB.
   * Root ssh access
-  * Python3 installed
+  * `python3-pip` installed. On a debian based system:
+
+    `apt install python3-pip`
 
 * A trusted local machine to run the installer on (we call this the `provisioning
   machine`):
@@ -272,7 +274,7 @@ When the installation is completed, you will have access to these applications:
     user. Its password can be found in
     `clusters/my-cluster/secrets/rocketchat_admin_password`.
 * [Grafana](https://grafana.com) that shows you information about the status of
-  your cluster. 
+  your cluster.
   - Read more about Grafana in the [monitoring chapter below](#monitoring)
 
 ### Known limitations