Allow enforcement of fallback solution

[smcvb]

Currently, the SpringCloudHttpBackupCommandRouter's knowledge of MessageRoutingInformation will only be used if no ServiceInstance metadata is present.

However, through some testing me and @corradom figured out that more recent versions of Eureka support metadata adjustment of the local Registration, but this knowledge is only propagated towards the DiscoveryClient once. Due to this, the global ServiceInstance metadata will be stuck on the command filter DenyAll, making it so that Eureka is currently unusable.

If we can enforce the MessageRoutingInformation from the SpringCloudHttpBackupCommandRouter to be used when such a discrepancy is in place, or if we enforce the usage of MessageRoutingInformation in favor of the ServiceInstance metadata, then we can alleviate this issue.

[smcvb]

Closing this issue as it has been resolved in pull request #24.