Move specification of minion-regexp to deploy-time rather than build-time.

Currently, /srv/pillar/common.sls is generated at build time and "instance_prefix" is set to whatever config-default.sh currently says it should be. This makes it impossible for the same release to be used to create clusters with different names.

This change moves the setting of "instance_prefix" to the deploy-time startup scripts (allowing $INSTANCE_PREFIX in config-default.sh to be changed without creating a broken cluster).

I've made corresponding changes in other providers where the flow was the same, but I have only tested on gce. Please feel free to yell at me if this breaks one of the other providers.