Support profile oriented style of config.

Support profile oriented style of config.

- support profile oriented style. It is compatible with subcommand
    oriented style.
- add a class Profile to store url/user/password of repos and api.
- add a class BizConfigManager extending from ConfigMgr. It
    handles profile info and does not care about multi-levels and
    passwdx issues. It returns current profile instance even
    config file has subcommand oriented style.
- make two functions decode_passwdx, encode_passwd to convert
    between passwd and passwdx.
- move get_repos_conf() from cmd_build.py to conf.py and split it
    into two parts
    _parse_build_repos() and _build_profile_by_subcommand()
- add test cases in test_profile.py

Change-Id: I81b2906456766f1061ccef9e7d51fea794c59409