provision: change the Provisioner interface
provision: change the Provisioner interface

 * Added the method RemoveUnits, that takes the app and the number of
   units to remove
 * Changed the method RemoveUnit. Now it takes the unit to remove, not
   the application

Getting rid of a bug introduced by tsr 0.5.0, with unit-add and
unit-remove.

cgvarela/tsuru,guilhermebr/tsuru,firebitsbr/tsuru,ArxdSilva/tsuru,tsuru/tsuru,hcxiong/tsuru,RichardKnop/tsuru,guilhermebr/tsuru,firebitsbr/tsuru,hugoantunes/tsuru,Endika/tsuru,cezarsa/tsuru,kgrvamsi/tsuru,cgvarela/tsuru,RichardKnop/tsuru,RichardKnop/tsuru,4eek/tsuru,ggarnier/tsuru,ggarnier/tsuru,tsuru/tsuru,gwmoura/tsuru,mderaldo/tsuru,gciotta/tsuru,cezarsa/tsuru,tuxmonteiro/tsuru,4eek/tsuru,tuxmonteiro/tsuru,gwmoura/tsuru,hugoantunes/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,4eek/tsuru,Endika/tsuru,tuxmonteiro/tsuru,gciotta/tsuru,ArxdSilva/tsuru,tsuru/tsuru,cgvarela/tsuru,mderaldo/tsuru,jblancett/tsuru,cezarsa/tsuru,kgrvamsi/tsuru,mderaldo/tsuru,gwmoura/tsuru,jblancett/tsuru,kgrvamsi/tsuru,ggarnier/tsuru,ggarnier/tsuru,hcxiong/tsuru,hcxiong/tsuru,gciotta/tsuru,cezarsa/tsuru,tsuru/tsuru,jblancett/tsuru,ArxdSilva/tsuru,firebitsbr/tsuru,Endika/tsuru,guilhermebr/tsuru