Add to multi-tenant swift store documentation

Make the usage of the 'swift_store_admin_tenants' parameter
a little clearer.

Fixes bug 1110597.

Change-Id: Ibcfe5d74b89e35ac65d442ebfa6f75b0f8519af4
Mark password config options with secret

Config object supports masking values when writing out if the secret
flag is set on the option definition.  This change flags all glance
options containing a password.

Change-Id: Id8428fbd5c0090840c8e9adf86badb794cf9fd66
Merge "Add to multi-tenant swift store documentation"
Merge "Mark password config options with secret"