Synnefo

Make astakos URLs configurable

Refs: #3351
Refs: #3340

• Define a general astakos URL prefix
  (ASTAKOS_GENERAL_URL_PREFIX setting) that precedes
  all the astakos URLs. By default is '/'.
• Define a views astakos URL prefix
  (ASTAKOS_VIEWS_URL_PREFIX setting) that precedes
  all the astakos views URLs. By default is
  `urlparse.urljoin(ASTAKOS_GENERAL_URL_PREFIX, 'im/').
• Define an api astakos URL prefix
  (ASTAKOS_API_URL_PREFIX setting) that precedes all
  the astakos api calls. By default is
  `urlparse.urljoin(GENERAL_URL_PREFIX, '')`
• Permit api call URL override.
• Expose authenticate, get_services, get_menu
  calls also under the api URL prefix. Currently
  they are also available under /im prefix.
  Maybe consider in the the future to keep them only
  under api URL prefix.

Note that possible changes to the default astakos
URL configurations may require the appropriate
changes also to the following settings:

• CLOUDBAR_SERVICES_URL
• CLOUDBAR_MENU_URL
• PITHOS_AUTHENTICATION_URL
• PITHOS_USER_CATALOG_URL
• PITHOS_USER_FEEDBACK_URL
• PITHOS_USER_LOGIN_URL
• PITHOS_UI_LOGIN_URL
• PITHOS_UI_FEEDBACK_URL
• UI_LOGIN_URL
• UI_LOGOUT_URL
• ASTAKOS_URL

astakos: WIP move all api calls under ``/astakos/api``

Refs: #3340