[RFE] Keystone to honor the "domain" attribute mapping rules

Problem Description
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Currently, Keystone identity provider (IdP) attribute mapping schema only uses the "domain" attribute mapping as a default configuration for the domain of groups being mapped; groups can override the default attribute mapping domain by setting their specific domain. However, there are other "elements" such as user and project that can also have a domain to define their location in OpenStack.

An operator when reading the attribute mapping section and seeing the schema for the attribute mapping definition, can be led to think that the domain defined in the mapping will also apply to users and projects. However, that is not what happens.

Proposed Change
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First of all, to facilitate the development and extension concerning attribute mappings for IdPs, we changed the way the attribute mapping schema is handled. We introduce a new configuration `federation_attribute_mapping_schema_version`, which defaults to "1.0". This attribute mapping schema version will then be used to control the validation of attribute mapping, and also the rule processors used to process the attributes that come from the IdP. So far, with this PR, we introduce the attribute mapping schema "1.1", which enables operators to also define a domain for the projects they want to assign users. If no domain is defined either in the project or in the global domain definition for the attribute mapping, we take the IdP domain as the default.

Moreover, we propose to extend Keystone identity provider (IdP) attribute mapping schema to make Keystone honor the `domain` configuration that we have on it. Currently, that configuration is only used to define a default domain for groups (and then each group there, could override it). It is interesting to expand this configuration (as long as it is in the root of the attribute mapping) to be also applied for users and projects.