provision: add Commandable interface
provision: add Commandable interface

This interface will be used for provisioners that want to add new
commands to the tsr command line utility.

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