more python/pmrep issues with string.replace("foo", None)

Hi Marko, I've tracked down the apparent cause of the segfaults I'm seeing
occasionally to another case of somestring.replace(None), which newer
python3 doesn't seem to like, e.g. :

$ python3 ./pmrep.py -s 1 -i '"1 minute",wrong,"5 minute"' --archive
./archives/20130706 -z -O 30m kernel.all.load,,"1 minute",,,16
Traceback (most recent call last):
  File "./pmrep.py", line 1615, in <module>
    P = PMReporter()
  File "./pmrep.py", line 188, in __init__
    self.config = self.set_config_file()
  File "./pmrep.py", line 383, in set_config_file
    conf = conf.replace("$PCP_SYSCONF_DIR", os.getenv("PCP_SYSCONF_DIR"))
TypeError: Can't convert 'NoneType' object to str implicitly

In this case os.getenv("PCP_SYSCONF_DIR") is returning None. This sometimes
ends up with a segfault, but not always (haven't tracked down why exactly).

A reasonable (?) patch seems to be to use the @static method
pmGetConfig('PCP_SYSCONF_DIR') here instead, which will still honor that
variable if it's in the environment, else get it from /etc/pcp.conf, which
resolves to /etc/pcp on most platforms, and so by default we properly load
/etc/pcp/pmrep/pmrep.conf.

diff --git a/src/pmrep/pmrep.py b/src/pmrep/pmrep.py
index 2168a54..fd2fb8c 100755
--- a/src/pmrep/pmrep.py
+++ b/src/pmrep/pmrep.py
@@ -380,7 +380,7 @@ class PMReporter(object):
         config = DEFAULT_CONFIG[0]
         for conf in DEFAULT_CONFIG:
             conf = conf.replace("$HOME", os.getenv("HOME"))
-            conf = conf.replace("$PCP_SYSCONF_DIR",
os.getenv("PCP_SYSCONF_DIR"))
+            conf = conf.replace("$PCP_SYSCONF_DIR",
pmapi.pmContext.pmGetConfig("PCP_SYSCONF_DIR"))
             if os.path.isfile(conf) or os.access(conf, os.R_OK):
                 config = conf
                 break