Passphrase creation fix

Currently the ```cylc reregister``` command deletes the suite passphrase when it unregisters the old name, but it does not create a new one when it registers the new name.  This unfortunately breaks the user guide tutorial section somewhat, as it recommends re-registering the example suites. 

This pull request fixes the problem by moving automatic passphrase creation from the register command into the registration library module where it should be (automatic deletion on unregistration is already there).

@matthewrmshin - please review.