Support schema types with non-str value

Change I75da1e9309e0f7ef8839dea3ec9c99c58edc5d63 introduced some
properties' types which are not string. This broke the `schema_args`
utility since lists are not hashable and there was no support for such
type values.

This patch fixes this issue with a very glance specific strategy in
which this values are assumed to have a `null` type and another type -
string, integer, etc. The fix ignores `null` options and it takes the
first non-null type as the valid one.

The patch adds support for enum types that accept `None`

Closes-bug: #1401032
Change-Id: I250e8912aca262a56c54ac59bb24f917e5d8cfce
Support Pagination for namespace list

The rest api metadefs/namespaces supports pagination using the parameters
of limit, marker, sort_dir, & sort_key. However, the glance client isn't
passing those parameters through (they come in as kwargs). This is
preventing pagination from working properly in horizon.

This is affecting Horizon support: https://review.openstack.org/#/c/104063/

Change-Id: Ib349cf3a3a437eb1711f350b37d0bd0e7d01330a
Closes-Bug: 1381816
Add release notes for 0.15.0

This adds release notes for the folowing commits:

    $ git log --oneline --no-merges 0.14.2..HEAD
    9a4d858 Support Pagination for namespace list
    3989cd2 Support schema types with non-str value
    9829d7b Don't require version to create Client instance
    8e02b2a Allow --file in image-create with v2 Image API
    d5a0e65 Add useful error on invalid --os-image-api-version
    49f38a4 Add release notes for 0.14.0 - 0.14.2
    5080d10 Send `identity_headers` through the wire
    d0851c3 Remove readonly options from v2 shell commands
    4194a55 Add --property-filter option to v2 image-list
    ef0abdc Fix py34 failure for glance client
    8c159a2 Don't set X-Auth-Token key in http session header if no token provided
    b83a672 Refactor method of constructing dicts in some tests
    751e064 Adds tty password entry for glanceclient
    597da8a '--public' ignored on image create
    cbbfbc9 Fix to ensure endpoint_type is used by _get_endpoint()
    12a3a82 Add bash completion to glance client

Change-Id: I24fbba80831c311b42d1abb08487cc29c8b5b162