Add keys validation testcase in substitution_mapping class

Add keys validation testcase in substitution_mapping class

Add keys validation in class of substitution_mapping according to
specification of
http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.0/TOSCA-Simple-Profile-YAML-v1.0.html:
1) Substitution mapping only supports keys of node_type, capabilities
and requirements;
2) The key of node_type is required, the others are optional.

JIRA:PARSER-80

Change-Id: Icd3284349175429e5ba5e52814819a6790f0e831
Signed-off-by: shangxdy <shang.xiaodong@zte.com.cn>