FEAT: add function to setup config search order.
ENH: consolidate default config file setting.

We remove most 'policy' choices from enstaller.config.Configuration:

  - Configuration write, _change_auth and _checked_change_auth now
    require an explicit filename
  - Configuration._get_default_config cannot create a new config file
    anymore. One should use write_default_config explcitly instead.
REF: put main-related tests into functional test suite.
BUG: fix functional test suite.
Merge pull request #164 from enthought/consolidate_default_config_location_handling

Consolidate default config location handling