Allow specifying the base_url in constructor of Resource subclasses and in CliBuilder

[djmason]

When using a non-default realm of signalfx you need to change the API base URL to be able to use it.

Right now the only way to change the base URL in any object is to directly set the base_url property on it, since the constructors do not propagate the Resource constructor's option to set the base_url.

Further to this, it's not possible to override the base_url using the CLI.