Makefile: Add target "docker_bootstrap_pull".
Makefile: Add target "docker_bootstrap_pull".

This simplifies downloading all bootstrap images at once.

I've also improved the log message for building and pushing the bootstrap image.

enisoc/vitess,mattharden/vitess,tirsen/vitess,rnavarro/vitess,tirsen/vitess,HubSpot/vitess,mapbased/vitess,sougou/vitess,applift/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,pivanof/vitess,vitessio/vitess,mapbased/vitess,tirsen/vitess,mahak/vitess,vitessio/vitess,HubSpot/vitess,mapbased/vitess,tirsen/vitess,applift/vitess,sougou/vitess,applift/vitess,applift/vitess,alainjobart/vitess,applift/vitess,sougou/vitess,applift/vitess,mattharden/vitess,tirsen/vitess,dcadevil/vitess,rnavarro/vitess,mattharden/vitess,dcadevil/vitess,pivanof/vitess,tinyspeck/vitess,rnavarro/vitess,tirsen/vitess,mapbased/vitess,mattharden/vitess,pivanof/vitess,rnavarro/vitess,dcadevil/vitess,mattharden/vitess,HubSpot/vitess,sougou/vitess,sougou/vitess,pivanof/vitess,davygeek/vitess,davygeek/vitess,mahak/vitess,dcadevil/vitess,HubSpot/vitess,mattharden/vitess,pivanof/vitess,rnavarro/vitess,tirsen/vitess,vitessio/vitess,tinyspeck/vitess,enisoc/vitess,mahak/vitess,enisoc/vitess,enisoc/vitess,sougou/vitess,pivanof/vitess,davygeek/vitess,alainjobart/vitess,enisoc/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,applift/vitess,davygeek/vitess,davygeek/vitess,alainjobart/vitess,sougou/vitess,mattharden/vitess,rnavarro/vitess,vitessio/vitess,alainjobart/vitess,dcadevil/vitess,alainjobart/vitess,rnavarro/vitess,dcadevil/vitess,tirsen/vitess,mapbased/vitess,tinyspeck/vitess,alainjobart/vitess,alainjobart/vitess,applift/vitess,mahak/vitess,mattharden/vitess,HubSpot/vitess,pivanof/vitess,mapbased/vitess,mapbased/vitess,davygeek/vitess,mahak/vitess,sougou/vitess,pivanof/vitess,rnavarro/vitess,pivanof/vitess,mapbased/vitess,vitessio/vitess,tinyspeck/vitess,HubSpot/vitess,enisoc/vitess,davygeek/vitess,enisoc/vitess,HubSpot/vitess,alainjobart/vitess,mahak/vitess,mattharden/vitess,HubSpot/vitess,vitessio/vitess,applift/vitess,applift/vitess,mapbased/vitess,dcadevil/vitess,mattharden/vitess,rnavarro/vitess,tirsen/vitess,tinyspeck/vitess,mapbased/vitess