make arc install-certificate <uri> work if uri is specified

Summary: see title

Test Plan: ran arc install-certificate <uri> in a directory without an .arcconfig and it worked! ran arc install-certificate in a directory with an .arcconfig and it worked! ran arc install-certificate <uri> in a directory with an .arcconfig and noted it correctly overrode the .arcconfig

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2251

Differential Revision: https://secure.phabricator.com/D4332