Adds an ability to manage share metadata

Now on share creating, we can specify metadata and it
will be saved in database. Also support of setting,
deleting and updating metadata info was added.

Adds model for share metadata
Adds migration
Adds api extension for share metadata
Adds relative api functions

Partially implements bp manila-client-enhancements
Change-Id: Id421323dd9e947d536b577092f1875f178d9aba5