title: snakemake --use-conda with an already created envs
body: <p>I work on an SGE cluster where some tools are available with conda :</p>
  
  <pre><code>source activate unicycler-0.4.7
  </code></pre>
  
  <p>I can use conda on my snakemake workflow with yaml, but in this way, I will going to reinstall the environment.</p>
  
  <pre><code>name: unicycler
  channels:
    - bioconda
    - conda-forge
    - defaults
  dependencies:
    - unicycler=0.4.7
  </code></pre>
  
  <p>Is it possible to activate an existing environment?</p>
  
  <p>Thanks very much for your helps!</p>
  
  <p>Version :</p>
  
  <ul>
  <li>snakemake 5.4.0</li>
  <li>conda 4.6.14</li>
  </ul>
  
  <p>Best regards,</p>