Make some Info and NodeInfo attributes optional

[nickjer]

Make some Info and NodeInfo attributes optional. For example:

job_info.cpu_time
# => nil

Some of these attributes (in particular Info#submit_host, Info#cpu_time, and NodeInfo#procs for Slurm) can not be retrieved for a given resource manager. Setting them nil will make them easy to check for existence and let the app display a default value if it wants.

For example:

<li class="job-info">
  <%= content_tag :ul, "Job Id = #{info.job_id}" %>
  <%# Don't display list item if it doesn't exist %>
  <%= content_tag :ul, "Submit Host = #{info.submit_host}" if info.submit_host %>
  <%# Set a default value for list item if it doesn't exist %>
  <%= content_tag :ul, "CPU Time = #{info.cpu_time || "Not Supported"}" %>
</li>