Add greenlet tracing code in snf-common, useful for debugging stuck
gevent gunicorn workers.

If the environmental variable SYNNEFO_TRACE is set to a non-empty
string, the tracing code will install a SIGTRAP signal handler for the
process, after the Synnefo settings have been imported.

Sending a SIGTRAP signal to the process (eg a gevent gunicorn worker)
will then print the stack trace of each greenlet spawned within the
process to stderr.