Revert "demote f20 to experimental"

This reverts commit 466ec42a0023de24518e9e958270637efecdb992.

Since this commit, nodepool has gained a fairer allocator that should
allow better forward progress when contention arises [1].  This should
alleviate the issue of pending devstack jobs not running due to
insufficient fedora nodes under heavy load

[1] https://review.openstack.org/#/c/101110/

Change-Id: I33a3f59dd4601e5cddc3ebd180287d4d8c6fee4c
Merge "Revert "demote f20 to experimental""