arcyd-tester: support '--phab-uri' arg

Make it possible to test using Phabricator installs not at the default
location by accepting an arg to specify it.

Test Plan:
$ ./precommit.sh
TEMP: debug stuff
fixup! TEMP: aphront error message
fixup! WIP: multiprocess logging
arcyd-tester: add '--arcyd-user-email-cert' arg

Make the arcyd user configurable, for more flexible testing.

Test Plan:
$ ./precommit.sh
fixup! arcyd-tester: support '--phab-uri' arg
arcyd-tester: refactor, extract make_creds_fr...()

Test Plan:
$ ./precommit.sh
arcyd-tester: '--[alice|bob]-user-email-cert' args

Allow customization of the 'alice' and 'bob' user credentials so that
the test can be pointed at a completely separate instance.

Test Plan:
$ ./precommit.sh

Use all customization options
$ ./proto/arcyd-tester --phab-uri http://127.0.0.1/api/ \
    --arcyd-user-email-cert phab phab@server.test xnh5t... \
    --alice-user-email-cert alice alice@server.test 35yxu... \
    --bob-user-email-cert bob bob@server.test 6wcgq...