Changing labels from 'key' to 'getenv'

[nwstephens]

• Changes all references from 'key' to 'getenv', which is slightly more descriptive.
• Makes getenv case sensitive by removing 'toupper'.
• Changes error message in Connect to be more descriptive.

[kevinushey]

If we want something more descriptive than "key"; that is, we want to document this as specifically being tied to an environment variable, then I'd suggest calling it "envvar".

Also: are we sure we want to call this something other than "key"? What if we wanted to eventually look up "key" from something that wasn't an environment variable (e.g. some kind of special integration with keyring, or other future expansions in how we look up secrets on the system)

[nwstephens]

We decided to change key to tag. Rolling those comments into #226 and closing this down.