Bugfix: When collectors reload. If current path is changed. Maybe raise KeyError

When i use

```
make run
```
or

```
python ./bin/diamond --configfile=conf/diamond.conf -f -l
```

start Diamond. After config settings `collectors_reload_interval` is arrived. Raise this:

```
[2014-03-31 21:21:49,570] [MainThread] traceback: Traceback (most recent call last):
  File "./bin/diamond", line 324, in main
  File "/home/xx/Diamond/src/diamond/server.py", line 334, in run
    self.mainloop()
  File "/home/xxx/Diamond/src/diamond/server.py", line 409, in mainloop
    collectors_path = self.config['server']['collectors_path']
  File "/usr/lib/python2.6/site-packages/configobj.py", line 553, in __getitem__
    val = dict.__getitem__(self, key)
KeyError: 'server'
```
I found the reason for the exception:

After Reloading config. and load_config, but os.path.abspath(self.config['configfile']) has changed:

```
/home/xxx/Diamond/src/collectors/cephstats/conf/diamond.conf
```
In [cephstats.py](https://github.com/BrightcoveOS/Diamond/blob/master/src/collectors/cephstats/cephstats.py)'s line 6, it changed the current directory.