We set the letsencrypt_self_generate_tokens value to True in testing
which means the variable is valid and exists in testing. However, in
production this variable isn't set and doesn't ahve a default so we get:
The task includes an option with an undefined variable. The error was:
'letsencrypt_self_generate_tokens' is undefined
Fix this by setting the default value for this var to False. Also, add
it to the README of letsencrypt-request-certs as this is where it is
primarily used.
Change-Id: I862df6ea3ff7f3a1df2a088b04d230bb618aaa85
4346570a0b