Add plugin interface for appending to tempest config

This commit adds a new interface to the external plugin interface for
adding configuration options to the config file. This is accomplished
by adding a method to the abstract class which plugins can use for
registering additional opts on the tempest config object. Then on the
tempest side a method is added to the plugin manager class to call the
register function for each plugin. This is then called at the end of
tempest's normal register_opts() function.

Partially Implements: bp external-plugin-interface

Change-Id: I2a7915cd978c496091dcf2cbf9d6a89ecbd8c2aa
2 files changed