`reproman backend-parameters` should not include warnings for failed imports

[asmacdo]

I don't have docker or boto installed, and I dont need them. backend-parameters shouldn't scare the user.

[f006rq8@x01 reproman]$ reproman backend-parameters
Backend parameters for 'aws-condor'
  access_key_id: AWS access key for remote access to your Amazon subscription.
  image: AWS image ID from which to create the running instance.
  instance_type: The type of Amazon EC2 instance to run. (e.g. t2.medium)
  key_filename: Path to SSH private key file matched with AWS key_name parameter.
  key_name: AWS subscription name of SSH key-pair registered. If not specified, 'name' is used.
  region_name: AWS availability zone to run the EC2 instance in. (e.g. us-east-1)
  secret_access_key: AWS secret access key for remote access to your Amazon subscription.
  security_group: AWS security group to assign to the EC2 instance.
  size: Number of EC2 instances in the HTCondor cluster.
  user: Login account to EC2 instance.
2023-02-21 11:29:11,319 [WARNING] Failed to import resource: No module named 'boto3' [aws_ec2.py:<module>:11]. Similar backends: aws-ec2 [base.py:get_resource_class:75] 
2023-02-21 11:29:11,328 [WARNING] Failed to import resource: No module named 'docker' [docker_container.py:<module>:11]. Similar backends: docker-container [base.py:get_resource_class:75] 
No backend parameters for 'shell'
Backend parameters for 'singularity'
  image: Base image filename or url
Backend parameters for 'ssh'
  host: DNS or IP address of server or ssh_config nickname
  key_filename: Path to SSH private key file
  port: Port to connect to on remote host
  user: Username to use to log into remote environment