Make Gateway status check optional when using FetchValidGateway and FetchValidTargetRef

[mikenairn]

What

Update FetchValidGateway to make the checking of the status conditions on the target Gateway optional when deciding whether to return it or not.

Update TLSPolicy to use these common function see

Why

FetchValidTargetRef and FetchValidGateway currently expect that the gateway should have a programmed condition, but in the case of the TLSPolicy it might not be programmed because there is an invalid tls section that this policy would resolve.