CLOUDIFY-2454 changed context properties to return None if absent
CLOUDIFY-2454 changed context properties to return None if absent

xdegenne/cloudify-plugins-common,cloudify-cosmo/cloudify-plugins-common,codilime/cloudify-plugins-common,isaac-s/cloudify-plugins-common,isaac-s/cloudify-plugins-common,codilime/cloudify-plugins-common,cloudify-cosmo/cloudify-plugins-common,codilime/cloudify-plugins-common,isaac-s/cloudify-plugins-common,cloudify-cosmo/cloudify-plugins-common,geokala/cloudify-plugins-common