Run migrations from only the bootstrap host

This ensures database migrations only happen on one node.  It moves database
migration from post-configure.d to configure.d within individual service
elements and protects all 'db sync' call with an os-is-bootstrap-host check
until we have a better story regarding leadership checks.  Also removes all
redundant 'db sync' calls from the boot-stack element (they were not actually
being run there, anyway).

Change-Id: Idf3c642614699cf3f79aefd7434b3eb4983df1ae
Closes-bug: #1339047
Merge "Run migrations from only the bootstrap host"