software/theia: Select free ports automatically

Changes:
- Add comments and reorganise instance.cfg.in for clarity
- Select free ports instead of hardcoded ports
- Upgrade to slapos.core 1.6.5 to prefix forwarded requests

This makes it possible to recursively nest theias into theias.

See merge request nexedi/slapos!919

software/theia/buildout.hash.cfg

@@ -15,7 +15,7 @@
 
 [instance]
 filename = instance.cfg.in
-md5sum = 397a2e0ac0d7f8813e0b594af6785300
+md5sum = 4f119df2b2d71e5ec1fd04051fd8fff8
 
 [yarn.lock]
 filename = yarn.lock

stack/slapos.cfg

@@ -200,7 +200,7 @@ setuptools-dso = 1.7
 rubygemsrecipe  = 0.3.0
 six = 1.12.0
 slapos.cookbook = 1.0.171
-slapos.core = 1.6.4
+slapos.core = 1.6.5
 slapos.extension.strip = 0.4
 slapos.extension.shared = 1.0
 slapos.libnetworkcache = 0.20