conduit api: Update bugzilla.account.search API query to only return only BMO provider types

Summary: Currently bugzilla.account.search will return even local accounts such as 'admin', etc. and should filter those out. This was causing errors in the BMO extension code that make updates when revisions change.

Test Plan:
Using the conduit API bugzilla.account.search method, enter the admins PHID in the query parameters
and observe that the results do not include the same PHID.

Reviewers: davidwalsh, smacleod

Reviewed By: smacleod

Bug #: 1415328

Differential Revision: https://phabricator.services.mozilla.com/D206