[v6r12] Av140512-v6r12: to deal with Disk/Tape separation

Ref. on JIRA: https://its.cern.ch/jira/browse/LHCBDIRAC-261?filter=-1

In order to deal with the Disk/Tape separation I made a small change in 
DowntimeCommand, ResourceManagementClient and ResourceManagementDB

2 more remarks:
1. we need an additional field "gocdbServiceType" in the DowntimeCache table inside the corresponding RM DB to store 2 service types ("srm" or "srm.nearline")
2. DowntimeCommand is using a getStorageElementOptions method in DIRAC.ConfigurationSystem.Client.Helpers.Resources and this might be changed soon