don't validate required property of policies

Problem:
Tosca-parser validates the required parameter for node_template but does not
validate the required property of the policy like node_template.
In the case of policies it should also be verified in the same way.

Example:
I confirmed the action of tosca-parser if the required parameter of policies is missing.

1. edit `toscaparser/tests/data/policies/test_tosca_nfv_multiple_policies.yaml`
    ----
      ...
      policies:
        - SP1:
            type: tosca.policies.tacker.Scaling
            targets: [VDU1]
            properties:
              #increment: 1 // comment out increment (this is defined "required: true")
              cooldown: 120
              min_instances: 1
              max_instances: 2
              default_instances: 1
        ...
    ----

2. exec `tosca-parser --template-file=toscaparser/tests/data/policies/test_tosca_nfv_multiple_policies.yaml`

It should error such as following, but it is not occur any errors.
    ---
    MissingRequiredFieldError: "properties" of template "SP1" is missing required field "['increment']".
    ---

In case of node_templates, the following error is occured.
    ----
    toscaparser.common.exception.ValidationError:
    The input "toscaparser/tests/data/policies/test_tosca_nfv_multiple_policies.yaml" failed validation with the
    following error(s):

            MissingRequiredFieldError: "properties" of template "VDU1" is missing required field "['image']".
    ----