Code optimization
Code optimization

The following codes have been excuted in function '_validate_exists',
and the result has been returned. therefore it is not necessary to
excute the following code again.

get_env_template = env_temps.EnvTemplateServices.get_env_template
env_template = get_env_template(env_template_id)

Change-Id: I2aeb835b1cda1b30f4fffbbd293beb42efc78588

openstack/murano,openstack/murano