Report underlying integer for unknown capabilities
Report underlying integer for unknown capabilities

Ieb1ea2ab45d76fc838c11489c2b029d7a5295e0e added a fix to report unknown
capabilities as "CAP_UNDEFINED".  This change improves on that slightly
to use `str(c)` for undefined capabilities rather than losing
information.  This also better parallels the oslo.config "input"
behaviour in `priv_context.CapNameOrInt`

Change-Id: Ic59576a4eba7bb8d40aee9e3d383417386aa08aa

openstack/oslo.privsep