autoscale: always starts autoscale goroutine
autoscale: always starts autoscale goroutine

This commit removes the need to enable the autoscale using
the configuration file and only relies on the rules registered
for each pool.

Fixes #1456

cezarsa/tsuru,cezarsa/tsuru,ArxdSilva/tsuru,ggarnier/tsuru,tsuru/tsuru,guilhermebr/tsuru,guilhermebr/tsuru,cezarsa/tsuru,guilhermebr/tsuru,guilhermebr/tsuru,cezarsa/tsuru,ArxdSilva/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,ggarnier/tsuru,tsuru/tsuru,tsuru/tsuru,tsuru/tsuru