ability to monitor ABSOLUTE_PATH instead of auto-generated BASE_PATH

$> onedatastore show 1
DATASTORE 1 INFORMATION                                                         
ID             : 1                   
[...]
BASE PATH      : /vmfs/volumes/ds1/1
[...]
DATASTORE TEMPLATE                                                              
BASE_PATH="/vmfs/volumes/ds1/"
BRIDGE_LIST="esxi_host1 esxi_host2"
CLONE_TARGET="SYSTEM"
DISK_TYPE="FILE"
DS_MAD="vmfs"
LN_TARGET="NONE"
TM_MAD="vmfs"
TYPE="IMAGE_DS"

above settings doesn't work because:

/vmfs/volumes/ds1 had already been mounted by esxi host (not to /vmfs/volumes/1). Setting BASE_PATH to /vmfs/volumes/ds1 means vmfs/monitor monitors /vmfs/volumes/ds1/1 which doesn't show in "df" output, hence capacity is unknown which results in not being able to download an image from the marketplace -- which by the way the import process returns no disk space available error (something in that nature) which is not true.

setting ABSOLUTE_PATH to /vmfs/volumes/ds1 and this change fix this inability to calculate the capacity issue.

this change enables users to use premounted esxi datastores.