VCDriver - No longer returns uptime due to multiple hosts

The vCenter cluster will have multiple hosts.
The cluster does not have a uptime feature.
Returning uptime of one of the host/longest
running host, can be misleading to the user.
Hence raising NotImplementedError
Closes-Bug:#1313375

Change-Id: I70e5ab35b8944abd7b874762de2af5b01d6a7313
Merge "VCDriver - No longer returns uptime due to multiple hosts"