Refactor shares client init in Tempest plugin

Class 'AltManager' was removed from Tempest project as redundant.
See: https://github.com/openstack/tempest/commit/8a938ad1

To keep compatibility of Manila plugin for Tempest was implemented refactor,
that does following changes:

- Make use of 'Manager' class defined in Manila plugin for inheritance by
  'Altmanager' and 'AdminManager' classes.
- Remove redundant code, that duplicates logic from inherited 'Manager' class.

Change-Id: Ife174c7b6e49b20b812b02c17ca8051dd2fd6a27
Merge "Refactor shares client init in Tempest plugin"