Be able to download prod/dev images

[horia-delicoti]

[vipulgupta2048]

@rcooke-warwick Found an issue already filed for this. We will be proceeding to add a way to download and use images other than what's provided locally to testbots. The feature will be made available to users with the configuration file option of having the downloadType property being set in.

downloadType: 'local', 'cloud', 'staging'