Add cloud_id column to environment model

When an environment is created, we'll need to create something in the
CSP and keep track of the ID of the created thing.

atst/models/environment.py

@@ -15,6 +15,8 @@ class Environment(Base, mixins.TimestampsMixin, mixins.AuditableMixin):
     project_id = Column(ForeignKey("projects.id"), nullable=False)
     project = relationship("Project")
 
+    cloud_id = Column(String)
+
     @property
     def users(self):
         return [r.user for r in self.roles]