provision/docker/bs: retry pulling bs image, avoid intermitent errors
provision/docker/bs: retry pulling bs image, avoid intermitent errors

hcxiong/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,kgrvamsi/tsuru,ArxdSilva/tsuru,hcxiong/tsuru,cezarsa/tsuru,tuxmonteiro/tsuru,ggarnier/tsuru,gciotta/tsuru,cgvarela/tsuru,ArxdSilva/tsuru,tuxmonteiro/tsuru,gwmoura/tsuru,kgrvamsi/tsuru,tuxmonteiro/tsuru,guilhermebr/tsuru,cezarsa/tsuru,hcxiong/tsuru,tsuru/tsuru,ggarnier/tsuru,Endika/tsuru,tsuru/tsuru,kgrvamsi/tsuru,gciotta/tsuru,Endika/tsuru,tsuru/tsuru,gwmoura/tsuru,Endika/tsuru,gwmoura/tsuru,ggarnier/tsuru,cezarsa/tsuru,tsuru/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,guilhermebr/tsuru,cezarsa/tsuru,ggarnier/tsuru,gciotta/tsuru,cgvarela/tsuru,cgvarela/tsuru