pip freeze output has comments sometimes: ignore them

Basically ignore comments as comments

The output of pip freeze can contain lines like

    ## FIXME: could not find svn URL in dependency_links for this
    package:

These shouldn't log an error about being unable to parse a comment.
Especially since the message pip is trying to deliver to the user will
generally be about the following line, which isn't displayed in salt
output.

The current output looks like:
```
[DEBUG   ] stderr: Warning: cannot find svn location for TracDateField==3.0.0dev-r12118
Warning: cannot find svn location for TracSpamFilter==0.8.0dev-r11796
Warning: cannot find svn location for nevernotifyupdaterplugin==0.1.0-r12968
[ERROR   ] Can't parse line '## FIXME: could not find svn URL in dependency_links for this package:'
[ERROR   ] Can't parse line '## FIXME: could not find svn URL in dependency_links for this package:'
[ERROR   ] Can't parse line '## FIXME: could not find svn URL in dependency_links for this package:'
```

The DEBUG stderr output is both more informative and less alarming for no good reason.