Refactor public share test cases

The RBAC policy defaults to create
a public share or to update an existing
share to public have changed since the
Stein release. [1]

Let's stop assuming that regular users
can perform these actions, so that manila
can safely remove the policy rule deprecation
support for these APIs.

[1] https://review.opendev.org/634864
Change-Id: I2a647fd5871ef6bb7d1ab45db893a44a560bed72
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
5 files changed