Allow prev-format `initial cycle point` with prev `cycle point format`

This allows a cylc 6 suite to use an `initial cycle point` in a cylc 5 format
provided that the `cycle point format` is also in the cylc 5 format.

For example, the following suite.rc snippet now passes validation:
```
[cylc]
    cycle point format = %Y%m%d%H
[scheduling]
    initial cycle point = 2014080800 # no T
```

@hjoliver, please review.