add spack debug option

[cyrush]

When things go wrong in spack land, the first request from spack folks is the stack trace obtained with:

spack --debug --stacktrace <command>

We should have a spack_debug option that allows users to easily find this info vs hacking it in.

[chapman39]

Currently we only debug a couple of spack commands (install and spec). Thoughts on adding the following code to the spack_exe() function instead? @cyrush @white238

debug = ""
if self.opts["spack_debug"]:
    debug = "--debug --stacktrace "
# Append debug to spack command

Otherwise this issue could probably be closed.

[white238]

Yes please. I actually ran into this last week.