Allow SecurityGroups to be set to empty with GoCLI cmd.
Allow SecurityGroups to be set to empty with GoCLI cmd.

- Currently when we try running command "./photon tenant set_security_groups 'tenant-id-1' ''"
with empty securitygroups , the go-cli fails with InvalidSecurityGroupFormat Exception.
It is because we are doing  regexp.split which creates an empty array that has
a blank string as its first element when we pass empty securityGroups.

Change-Id: Ib2a8a8e7be13d8e59484a1ec50aabd86efb8c20f

vmware/photon-controller-cli,vmware/photon-controller-cli