Add ability to specify scheme of ServiceProviderConfiguration

[DavidLievrouw]

Add ability to specify scheme of ServiceProviderConfiguration, for use in the ConsulClientFactory

Recreation of #960 after request from @TomPallister.

Fixes / New Feature #959

In the ConsulClientFactory class, the scheme of the Consul server address is hard-coded to "http". In our environment, Consul is not hosted over http, only over https.

Proposed Changes

• Add property "Scheme" to ServiceProviderConfiguration.
• Use it in ConsulClientFactory, by adding it to ConsulRegistryConfiguration.
• Implement a backward compatibility fallback to "http".
• Make sure it is filled in when loading the GlobalConfiguration from file.

[TomPallister]

@DavidLievrouw sorry it has taken me so long to merge this PR.

Thank you very much for your contribution to Ocelot!!