admin: Make Admin base URL configurable

Add an `ADMIN_BASE_URL` setting, which works roughly the same as the
Cyclades/Astakos base URLs (minus the service registration part). The
main benefit of the `ADMIN_BASE_URL` setting is that it allows the
administrator to define the path for Admin (or lack thereof).

Also, using the `extends_path_with_slash` function, the base URL that
the administrator chooses should work with or without a slash. In the
latter case, a Django-specific redirect view will be used to redirect
the request to the URL with slash.

Finally, the addition of the `ADMIN_BASE_URL` brings also another
change. If the administrator wants to install Admin in a separate node,
he/she no longer has to choose between using as ROOT_URLCONF the
'synnefo_admin.urls' or 'synnefo_admin.admin.urls' (their main
difference was that the former had '/admin/' hard-coded).
Using simply the 'synnefo_admin.urls' should suffice,
since the `BASE_PATH` will be extracted from the `ADMIN_BASE_URL`.

Fixes apyrgio/synnefo#289
admin: Add URL test for Admin

Add a test that checks if the `ADMIN_BASE_URL` setting is used by Admin
and if we have any issues with slashes and redirections.
docs: Update install/admin guides

Update the install/admin guides to reflect the addition of the
`ADMIN_BASE_URL` setting. Also, add an Admin section in the CentOS
install guide.
deploy: Add `ADMIN_BASE_URL` in Admin conf

Add an `ADMIN_BASE_URL` setting in the admin.conf file. The value of
this setting is created in the same fashion as the values for
CYCLADES_BASE_URL / ASTAKOS_BASE_URL.
Mention in Changelog the `ADMIN_BASE_URL`