Checking if file exists and always use abspath

It should not break any of implemantation, but it needs to be tested in all services which are using it. Here are changes not to check `if pfn.startswith('/')` but to check `if isfile(pfn)`

In any case, if any of these functions will get absolute path or relative path everything should work :

```
>>> import os
>>> file = 'cmsRun-stderr.log'
>>> if os.path.isfile(file):
...     print os.path.abspath(file)
... 
/tmp/condor/execute/dir_20810/glide_Th7adb/execute/dir_25952/cmsRun-stderr.log
>>> file = os.path.abspath('cmsRun-stderr.log')
>>> if os.path.isfile(file):
...     print os.path.abspath(file)
... 
/tmp/condor/execute/dir_20810/glide_Th7adb/execute/dir_25952/cmsRun-stderr.log
```
@ericvaandering, @ticoann, @mmascher can you take a look ?

I will comment in this hypernews with more details soon : https://hypernews.cern.ch/HyperNews/CMS/get/crabDevelopment/2103.html