rebootstrap architecture update

 * arm is completely dead
 * ppc64el does not have multilib

This commit removes the following jobs:
rebootstrap_arm_gcc49 rebootstrap_ppc64el_gcc49_nobiarch
enable munin plugins relevant to io load
Merge remote-tracking branch 'helmut/rebootstrap'
munin only reloads with force
Merge remote-tracking branch 'helmut/munin-io'