[Python experimental] Add __setattr__ function to ensure signing_info.host is the same as configuration.host when the user assigns signing info

Created by: sebastien-rosset

To initialize the configuration, developers can write the following code:

configuration = Configuration(
   host = "http://example.com",
   configuration.signing_info = openapi.HttpSigningConfiguration(
     signing_scheme = intersight.signing.SCHEME_RSA_SHA256,
     ...
   )
)

Or, for whatever reason, they can first create the config object, then assign the signing info:

configuration = Configuration(
   host = "http://example.com"
)
...
configuration.signing_info = openapi.HttpSigningConfiguration(
   signing_scheme = intersight.signing.SCHEME_RSA_SHA256,
   ...
)

In the first case, the host parameter is automatically passed from the Configuration object to the HttpSigningConfiguration. In the second case, the HttpSigningConfiguration.host was None. This PR fixes the problem for the second case, i.e. the developer should not have to set the parameter twice.

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