Implement REST API microversions in Manila

The OpenStack API working group recommends all projects adopt Nova-style microversions for versioning their REST APIs. This commit ports the Nova microversion code and docs to Manila.

With this patch, the API version is bumped to 1.1, and the versions API (which has always returned horribly outdated values) is the first API that is versioned. The 1.1 version of the versions API includes the minimum and current API version values.

Here is the updated 1.0 version (without version fields):

curl -i -X GET http://localhost:8786/ -H "Accept: application/json" -H "User-Agent: python-manilaclient"

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 339
X-Openstack-Manila-Api-Version: 1.0
Vary: X-OpenStack-Manila-API-Version
Date: Thu, 06 Aug 2015 16:10:13 GMT

{"versions": [{"status": "CURRENT", "updated": "2015-07-30T11:33:21Z", "media-types": [{"base": "application/json", "type": "application/vnd.openstack.share+json;version=1"}], "id": "v1.0", "links": [{"href": "http://docs.openstack.org/", "type": "text/html", "rel": "describedby"}, {"href": "http://localhost:8786/v1/", "rel": "self"}]}]}

And here is the 1.1 version (with version fields):

curl -i -X GET http://localhost:8786/ -H "Accept: application/json" -H "User-Agent: python-manilaclient" -H "X-OpenStack-Manila-API-Version: 1.1"

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 379
X-Openstack-Manila-Api-Version: 1.1
Vary: X-OpenStack-Manila-API-Version
Date: Thu, 06 Aug 2015 16:11:51 GMT

{"versions": [{"status": "CURRENT", "updated": "2015-07-30T11:33:21Z", "links": [{"href": "http://docs.openstack.org/", "type": "text/html", "rel": "describedby"}, {"href": "http://localhost:8786/v1/", "rel": "self"}], "min_version": "1.0", "version": "1.1", "media-types": [{"base": "application/json", "type": "application/vnd.openstack.share+json;version=1"}], "id": "v1.0"}]}