warn about merged-usr in ganeti, disable it in robot manual

tsa/howto/ganeti.mdwn

@@ -38,6 +38,9 @@ See also the more extensive [glossary in the Ganeti documentation](http://docs.g
 
 ## Adding a new instance
 
+WARNING: this creates a machine with usrmerge! See [bug 34115](https://trac.torproject.org/projects/tor/ticket/34115)
+before proceeding.
+
 This command creates a new guest, or "instance" in Ganeti's
 vocabulary:
 

tsa/howto/new-machine-hetzner-robot.mdwn

@@ -123,6 +123,7 @@ which we'll use below.
         mkdir /target/run/udev && mount -o bind /run/udev /target/run/udev &&
         apt-get install -y grml-debootstrap && \
         grml-debootstrap \
+            --debopt "--no-merged-usr" \
             --grub "$BOOT_DISK" \
             --target /target \
             --hostname `hostname` \