Update export location retrieval APIs

Update the export location retrieval APIs for shares and share instances to do the following:

1. Restore the API-to-view-builder calling convention of index-->summary and show-->detail. In so doing, modify which values are returned by the list commands (currently, all but the timestamps). The admin context from the request determines whether the admin-only values are returned.

2. Report the UUID field from the export location table as 'id' to be consistent will all other objects returned via the Manila REST API.

3. Add the preferred flag to the output of the API. Drivers can report preferred:True or preferred:False in their export location metadata, and this standard flag will be returned via the REST interface, like this:

+-------------------+--------------------------------------+
| Property | Value |
+-------------------+--------------------------------------+
| is_admin_only | False |
| uuid | df828d44-0b04-47fa-8ee5-516ffc199ca7 |
| share_instance_id | 1b40e873-331e-4e1c-ab53-38ec95b3bfcc |
| path | 10.0.0.100:/share_1b40e873 |
| created_at | 2016-02-18T21:12:51.000000 |
| updated_at | 2016-02-18T21:12:51.000000 |
| preferred | True |
+-------------------+--------------------------------------+

Ideally, we could do some of this cleanup in multiple commits, but it is advantageous to do so in a single commit in order to do it in a single microversion bump. It is also more expedient at this time in the Mitaka cycle.