Upgrades: initialization command/snippet

This allows to run a command or a script snippet on all overcloud nodes
at the beginning of the upgrade. The intended use is to switch to a new
set of repositories on the overcloud. This is done differently in
different contexts (e.g. upstream vs. downstream), but generally it
should be simple enough to not warrant creation of switchable
"UpgradeInit" resource in the resource registry, and a string
command/snippet parameter should suffice.

Change-Id: I72271170d3f53a5179b3212ec9bae9a6204e29e6

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