Now the test runners use the LD_PRELOAD environment variable to load
the correct libstdc++ built by slapos and restart themselves.

The restart is necesasry because LD_PRELOAD is only taken into account
at the time the process is started. Changing this environment variable
in runtime has no effects.

Slapos.toolbox has a new dependency to pycurl, which fails to compile if libcurl cannot be found. This commit adds the component/pycurl dependency to most of the Software Releases which have a dependency to slapos.toolbox.

I tried to do it globally, so I couldn't check the compilation of all the modified SRs. Is it a way to test that all SRs still compiled ?

Also, did I forget some important SR ?

@rafael @alain.takoudjou @kazuhiko @jm 

/reviewed-on nexedi/slapos!97

The recipe userinfo already provides this information

This reverts commit fd5f808b.