If master_api_key set, allow it to run_as if any user can run_as.
If no master_api_key set no change in behavior, if api_allow_run_as is empty no change in behavior (i.e. run_as will not work with master_api_key if no api_allow_run_as users are configured - as to not increase attack surface for these Galaxy instances).

This changeset contains no tests, but pull request #278 included functional tests for these changes (as well as original behavior) which passed.
Merged in jmchilton/galaxy-central-fork-1 (pull request #288)

If master_api_key set, allow it to run_as if any user can run_as.
Remove redundant run_as_users check.