Import gpg key in get_images(), not get_signing_key().

Was not being imported because was lumped in with get_signing_key() step
which only occurs on fresh spoke AND fresh hub. Failed on other
combinations.
Use proper caps for "True" and "False" checking.

Python in fig makes true/false statements in the fig.yml proper True and
False. I was checking for lowercase true/false which could only happen
if the setting was an actual string (ie "true", not True).
Add message to alert when $REFRESH_IMAGES is about to update.