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