Document how to use CRAB on CMSSW_7_x

[belforte]

problems:

1. needs to use CMSSW_CC6 container
2. but that container does not have myproxy

• needs to be in an SL6 container
• needs to use a shell window in SL7 to issue crab createmyproxy at least once/week
• need to create a valid proxy in the SL6 contariner

  voms-proxy-init -voms cms -rfc -valid 172:00
  export X509_USER_PROXY=`voms-proxy-info -path` 

• needs to use all crab command inside the container as

  crab <command>  --proxy=$X509_USER_PROXY .....

possible imporovement: depending on usage/demand, make crab client detect that myproxy-* is missing and fall back automatically the crab <command> --proxy=$X509_USER_PROXY mode

[belforte]

done in https://twiki.cern.ch/twiki/bin/view/CMSPublic/CRAB3FAQ#I_need_to_use_a_old_CMSSW_releas