config: reduce calls to findall.

This call is slow and is called many times which can seriously slow down
commands such as `cylc validate`. This change uses 2 str tests to see if
there is a any need to call the findall command at all.

A user's reported that his suite was taking 40s to validate. I have identified 3 bottlenecks:
1. This findall call contributes to around 13s or so of the time.
2. Another 15s or so are spent doing database select, which we are going to send another pull request to fix.
3. It also spends around 5s or so calling `cycon.next`, which I'll investigate further.)