The different commands in RBTools have lots of differerent needs. Some

of them interact only with your local repository, some of them interact

only with the server API, some do both, and some do neither. We've

traditionally had a few methods to handle things like setting up the API

or the SCMClient, but usage of these was inconsistent and resulted in a

lot of repeated boilerplate. Because of this and the history of RBTools

development, usage of all of this was somewhat inconsistent, with many

commands doing more or less than they really needed to do.
This change is the first of a couple to clean this up. Two new

attributes have been added to the base Command class: needs_api and

needs_scm_client. Subclasses can set these to True depending on

their needs, and then prior to main being called, the initialization

process will set various attributes: api_client, api_root,

capabilities, repository_info, server_url, and tool.
This change handles most of the initialization process. The one major

remaining piece are the calls to find_server_repository_info, which

will require some more involved changes (for example, short-cutting in

the case when REPOSITORY_NAME or REPOSITORY_URL are defined).

Smoke tested basic functionality for all changed commands.
Verified rbt patch behavior was correct with both ID and full URL

  (ID only and ID + diff revision) arguments. Checked that

rbt patch --print did not initialize any repository support.
Verified rbt land behavior was correct with both ID and full URL

--review-request-id option.
Checked that rbt status --all did not initialize any repository

  support.