adapt to SL6 container

[belforte]

when running inside cmssw-cc6 should simply skip calls to myproxy or gfal and print proper warnings instead.
Should manage to make it work in SL6 w/o the need to use --proxy option.

[belforte]

a way to detect where we are running, in the env:
SINGULARITY_NAME=slc6:x86_64

[belforte]

not really that high priority !

[belforte]

better solution

• only allow crab submit and crab status commands. For everything else tell users to use el7 or newer container