Silence [CRITICAL] Specific ext_pillar interface qvm_prefs is unavailable errors

[eloquence]

#530 added logic to aggressively clear the Salt cache before installing/uninstalling. This in turn appears to have introduced the following line to the provisioning output:

[CRITICAL] Specific ext_pillar interface qvm_prefs is unavailable

Any line that contains the word critical in all-caps is likely to be alarming to the person performing the installation, so we should attempt to either suppress this output, or implement conditional checks to prevent it.