Build: allow to build only py2 rpms for fedora

This is more or less for testing purposes of py2/py3 compatibility

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

BUILD.txt

@@ -36,6 +36,11 @@ It may be possible to do a simple make install but this has not been
 well-tested. Additional work is done in pre/post install scripts in the ipa
 spec file.
 
+To build only python2 packages on fedora following steps are required:
+$ autoreconf -i
+$ ./configure
+$ make rpms RPMBUILD_OPTS="--define 'with_python3 0'"
+
 Developing plugins
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