Support variable session length for Rococo chains at genesis (#2167)

This pr adds support to change the session length of a Rococo chain at
genesis. This is rather useful because Rococo has a session length of
1 hour, while on rococo-local you will now get 1 minute. This improves
the dev experience, because a parachain is only going live at the
start of a new session.
10 jobs for master in 17 minutes and 24 seconds (queued for 1 second)