Sort the ecode list in the gnt-cluster manual for deterministic results.

Submitted upstream: <https://github.com/ganeti/ganeti/pull/1504>.

diff --git a/lib/build/sphinx_ext.py b/lib/build/sphinx_ext.py
--- a/lib/build/sphinx_ext.py
+++ b/lib/build/sphinx_ext.py
@@ -108,7 +108,7 @@ CV_ECODES_DOC = "ecodes"
 # pylint: disable=W0621
 CV_ECODES_DOC_LIST = [(name, doc) for (_, name, doc) in constants.CV_ALL_ECODES]
 DOCUMENTED_CONSTANTS = {
-  CV_ECODES_DOC: CV_ECODES_DOC_LIST,
+  CV_ECODES_DOC: sorted(CV_ECODES_DOC_LIST, key=lambda tup: tup[0]),
   }
 
 
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<div class='path'>path: <a href='/guix/log/?id=87d37a02f7642129a6d225fcd77df41cb314ab40&amp;showmsg=1'>root</a>/<a href='/guix/log/etc?id=87d37a02f7642129a6d225fcd77df41cb314ab40&amp;showmsg=1'>etc</a>/<a href='/guix/log/etc/historical-authorizations?id=87d37a02f7642129a6d225fcd77df41cb314ab40&amp;showmsg=1'>historical-authorizations</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/historical-authorizations?id=87d37a02f7642129a6d225fcd77df41cb314ab40'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2020-07-11 11:51:56 +0200'>2020-07-11</span></td><td class='logsubject'><a href='/guix/commit/etc/historical-authorizations?id=4b9b8b2ec23e59af2275a6d2829e6e0f915b44b3'>Add 'etc/historical-authorizations'.</a></td><td>Ludovic Courtès</td></tr>
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* etc/historical-authorizations: New file.  Data extracted from
'build-aux/git-authenticate.scm'.
* Makefile.am (EXTRA_DIST): Add it.


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