Contributing.md should be updated to represent local testbed need correctly

[shuklanirdesh82]

https://github.com/vmware/docker-volume-vsphere/blob/master/CONTRIBUTING.md#build-test-rinse-repeat

see below

Test environment typically consist of 1 ESX and 2 guest VMs running inside of the ESX. We also support 1 ESX and 1 guest VM. We require ESX 6.0 and later, and a Linux VM running Docker 1.10+ enabled for plain text TCP connection, i.e. Docker Daemon running with "-H tcp://0.0.0.0:2375 -H unix:///var/run/docker.sock" options. Note that both tcp: and unix: need to be present Please check "Configuring and running Docker" (https://docs.docker.com/engine/admin/configuring/) page on how to configure this - also there is a github link at the bottom, for systemd config files.

With the current automated tests we have added some pre-requisite for the local testbed to be configured.

• swarm tests needs swarm cluster to be pre-configured (which requires 3 vms in the test bed)
• some tests requires two vms to be present

/CC @msterin