Added host power controls and associated tests 

Hosts class
* Added CommonPowerButton inheritance
* Added reset_host() method for reseting host
* Added power_on_host() method for powering on host
* Added power_off_host() method for powering off host

test_infrastructure_providers_pages
* Added test_host_power_controls_reset() test
* Added test_host_power_controls_on() test

Base class
* Added click() method to Flash message object