Clarify that RFE bug reports should not have an importance set

Clarify that RFE bug reports should not have an importance set

Filling the Importance field [1] with anything other than Wishlist for RFE bug reports
is wrong. People confuse priority with Severity (which is what the importance field is
about), and that can cause us some grief.

Therefore let's state clearly that from now on, RFE bug reports will be marked 'Wishlist'
only.

[1] https://wiki.openstack.org/wiki/Bugs#Importance

Change-Id: I4f12301e5d6b9b754efd456f0fb9da4e64555373