Add support for showing and sorting by additional fields in "server list"

This PR adds two additional switches to ```server list``` command:
```
--show field1,field1,.       Show additional fields. Supported: compute_node, tags
--sort field                 Sort by field, default is name, supported also: compute_node, price
```
Previous version would optionally show tags when environment variable ```SHOW_TAGS``` was set, which 
was incompatible with flag-based configuration.  This functionality is now replaced by ```--show tags``` switch.

When sorting by ```compute_node``` we add an extra line break between zones, to group zones sharing
a single compute node together. You can sort by ```compute_node``` without showing it.