Fetch list of SA sources from Emulab. Use magic to get names/urls for now.
Add support for custom URLs to fetch certificate from MAs.
Permit a tool to specify an MA.

If a tool knows what MA the user should use to get their certificate,
pass that hint along to the signer tool. The signer tool will display
that MA as the default or append it to the list if it isn't in the set
of MAs.
Merge branch 'develop' into default-ma
Merge branch 'default-ma' into develop