core: Remove redundant Vds call at Add existingFileStorageDomain

Remove redundant getStorageDomainsList VDS call.
The call was used to know if the Storage Exists in the VDSM, but we can
also know that from the next VDS command of getStorageDomainInfo.
Changing the test accordingly

Change-Id: I2294f83eb204cab44b6a52e7c7a763bf107aaa37
Bug-Url: https://bugzilla.redhat.com/1157240
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>