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


git-svn-id: svn://svn.forge.objectweb.org/svnroot/contrail/trunk/conpaas@5118 ef35cc4a-2640-4bbf-98bc-65920db7d8ae
conpaas-core: add unit tests for conpaas.core.clouds.base


git-svn-id: svn://svn.forge.objectweb.org/svnroot/contrail/trunk/conpaas@5119 ef35cc4a-2640-4bbf-98bc-65920db7d8ae