Set environ config fields on bootstrap

Add environment info to config on bootstrap prepare. API endpoints will be added in a followup branch.
This change stops TestUserInfo, TestUserInfoFormatJson and TestUserInfoFormatYaml from failing. Each of these tests call testing.RunCommand(c, newUserInfoCommand()) which in turn calls client.UserInfo, passing in a user tag id (cmd/juju/user_info.go:103).

UserInfo.username should no longer contain an empty string, as it is populated with "admin" on bootstrap prepare.