Use 0-based indices for location entries

Glance server currently uses 1-based indices for indexing location
entries. This violates the JSON-pointer RFC (RFC 6901) which states that
indicies should be 0-based. Furthermore, since this deviation is not
mentioned in the docs any application directly calling the REST APIs
would trip over it.

This commit makes Glance use zero-based indexing for location entries.

Note that the "show_multiple_locations" config option should be set to
True to expose location related REST operations.

DocImpact: Since this change modifies user-visible Glance behaviour it
should be explicitly documented because the previous behaviour (1-based
indexing), though not explicitly documented, was exposed through the
REST APIs.

Change-Id: I6326455874c381fcb0d8babf9cc4fa8311e219d2
Closes-bug: #1282437
Merge "Use 0-based indices for location entries"