Declare testability in distributions.conf instead of hardcoding it

Mark buster and bullseye as not testable, too, for now.

Move comment about #659360 (debootstrap cannot build Ubuntu Edgy or
earlier) from release-testing to distributions.conf, too.

etc/distributions.conf

@@ -5,7 +5,7 @@
 # codename = distribution and further keywords
 #
 # Known distributions: debian, ubuntu
-# Known keywords: eol, pygrub, default-keyring
+# Known keywords: eol, pygrub, default-keyring, dont-test
 #
 sarge        = debian eol
 etch         = debian eol
@@ -14,8 +14,8 @@ squeeze      = debian eol default-keyring
 wheezy       = debian
 jessie       = debian
 stretch      = debian
-buster       = debian
-bullseye     = debian
+buster       = debian dont-test
+bullseye     = debian dont-test
 sid          = debian
 
 testing      = debian
@@ -24,8 +24,10 @@ oldstable    = debian
 stable       = debian
 unstable     = debian
 
-dapper       = ubuntu eol
-edgy         = ubuntu eol
+# dapper and edgy currently need manual adjustments in debootstrap's
+# configuration, see #659360
+dapper       = ubuntu eol dont-test
+edgy         = ubuntu eol dont-test
 feisty       = ubuntu eol
 gutsy        = ubuntu eol
 hardy        = ubuntu eol