conpaas-core: fix clouds.base.Cloud._check_cloud_params
conpaas-core: fix clouds.base.Cloud._check_cloud_params

This patch improves _check_cloud_params by:

- calling get_cloud_type() to generate a proper error message where appropriate
- making it more explicit


ConPaaS-team/conpaas,ConPaaS-team/conpaas,ConPaaS-team/conpaas,ConPaaS-team/conpaas,ConPaaS-team/conpaas,ConPaaS-team/conpaas,ConPaaS-team/conpaas