Added a plugin mechanism for arcyd

Summary:
A system for allowing use case specific code to be called from
within arcyd.

Test Plan: TODO

Reviewers: oparanal, jevripio, pmartin2

Reviewed By: jevripio

CC: phab

Differential Revision: https://sibld.phab.dev.bloomberg.com/D153
Added single test script

Summary:
A simple script to run individual tests.
Usage example: $ singletest.sh abdi_processrepo Test test_A_Breathing

Test Plan: None

Reviewers: jevripio

Reviewed By: jevripio

CC: phab

Differential Revision: https://sibld.phab.dev.bloomberg.com/D163
fixup! Use new Arcyd conduit, step towards testability

Summary: Conduit object gets passed in as a list which generates errors. This fixes it but may not be the prefered fix.

Test Plan: None

Reviewers: oparanal, jevripio

Reviewed By: jevripio

CC: phab

Differential Revision: https://sibld.phab.dev.bloomberg.com/D165
fixup! Arcyd: test branchmock iface matches real branch
Arcyd: test processrepo with a bad landing

Simulate a bad landing by monkey patching branch.land and raising a bad
landing exception.

Test Plan: $ ./precommit.sh
Arcyd: tidy comments in abdi_processrepo__t.py

Test Plan: $ ./precommit.sh
Arcyd: tidy conduit concerns into test driver

Test Plan: $ ./precommit.sh
Arcyd: add empty abdt_branch__t and move concerns

Test Plan: $ ./precommit.sh