Registered ipcm variable can be checked against
stat.exists in order to assert existence of ipcm

I found that include paths of slapos package's perl were wrong (i.e. `/usr/src/packages/BUILD/SLAPOS/build` should be removed).

```
~> /opt/slapos/parts/perl/bin/perl -e 'print @INC,"\n"'
/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/site_perl/lib/site_perl/5.20.2/x86_64-linux-thread-multi/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/site_perl/lib/site_perl/5.20.2/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/perl/lib/5.20.2/x86_64-linux-thread-multi/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/perl/lib/5.20.2.
```

I think that this MR fixes this issue but not so sure. Can you please verify it ?

/cc @alain.takoudjou @rafael 

/reviewed-on nexedi/slapos.package!23

because for Debian, 0.479+slapos.gfd8673b is newer than 0.479+slapos2.g12d4231

  The implementation was moved to slapos master as a website.

 OpenSSL is broken inside rebootstrap folder as it cannot find appropriate
 openssl.cnf . This should be solved at buildout level however it might
 require way too much hours of debug. Please revert this change as soon
 buildout includes appropriate fix.

It's all the more important as the IPCP often crashes when the address is
unknown during enrollment. See https://github.com/IRATI/stack/issues/872