Renewing a slice at CH2 endpoint cares about credential order
If the slice credential is not the *first* credential passed to `update` for a slice, you get the following error:
```
<?xml version="1.0"?>
<methodResponse>
<params>
<param><value><struct>
<member><name>protogeni_error_log</name><value><string>urn:publicid:IDN+emulab.net+log+6d076e2fe19010ab7441715a5befc613</string></value></member>
<member><name>protogeni_error_url</name><value><string>https://www.emulab.net/spewlogfile.php3?logfile=6d076e2fe19010ab7441715a5befc613</string></value></member>
<member><name>value</name><value><i4>0</i4></value></member>
<member><name>output</name><value><string>You gave a credential for urn:publicid:IDN+emulab.net+authority+sa but tried to update another slice: urn:publicid:IDN+emulab.net:ig-control-of+slice+scratch-a</string></value></member>
<member><name>code</name><value><i4>3</i4></value></member>
</struct></value>
</param>
</params>
</methodResponse>
```
issue