Make Murano stats panel visible to admin only

In the absence of RBAC implementation, make the stats panel accessible
only to users with role 'admin', as with other horizon components.

Change-Id: I5231e682796d83625c4fbde294c7ad2289e1e55d
Implements: blueprint stats-panel-visible-to-admin-only
Merge "Make Murano stats panel visible to admin only"