Users can currently run `dep_env_create/delete` workflows while other system workflows are running.
The reason is that when running one of these commands we don't verify whether other workflows are running (see https://github.com/cloudify-cosmo/cloudify-manager/blob/master/rest-service/manager_rest/resource_manager.py#L539).
This means that you can run a snapshots and during the DB backup also run `cfy dep create` - a command that changes the state of the DB.
For some historical reason `_create_deployment_environment' and '_delete_deployment_environment' are system workflows, I think that before executing one of them we need to use the same verification any "regular" (deployment) workflows goes through.
Steps to reproduce:
1. Add a `sleep` to https://github.com/cloudify-cosmo/cloudify-common/blob/master/cloudify/dispatch.py#L603 if the execution is 'create_snapshot'
2.Run `cfy deployments create...`
The second command (`cfy deployments create...`) should raise an error (manager_exceptions.ExistingRunningExecutionError)
`cfy deployments create...` works while a snapshot is running