Support hiding preproduction cells

Add settings for hiding preproduction cells from the cell selection
tab unless a user has a certain role on the current tenant:

OPENSTACK_PREPROD_CELLS = ['cell1', 'cell2']
OPENSTACK_PREPROD_ROLE = 'PreProdUser'

Gbp-Pq-Topic: nectar
Gbp-Pq-Name: cell_permissions.patch