add stevedore dependency
add BaseAgentMode class
replace StandbyAgent with StandyMode
replace DecomAgent with DecomMode
register standby and decom modes in setup.cfg
allow BaseTeethAgent to load a mode implementation
rename BaseTeethAgent to TeethAgent
use a mode implementation in the teeth agent
unify the agent executables in teeth-agent
split out base and agent modules
update imports
update tests
clarify test module naming
add an OverlordAPIError
add an API client method to fetch agent config
load agent mode from the configuration API
update standby tests