shorten command to switch CLI config

[imogenkinsman]

Changing between CLI config is the most common config action, but currently that requires updating the config to active, like this:

influx config update -n my-config -a

We should make this way easier to remember, ideally no more than three words and no flags.

My preference is that we shorten it to this:

influx config my-name

To make my-name the active config.