Addition of waits and retries to have the instance vnc URL ready

[alessandroargentieri]

VNC Console access URL is not immediately ready and requires some retries.
This PR aims to perform the retries before redirecting the user to the default browser.
Manual test performed:

~/work/cli  $ go run main.go instance vnc 34e0e710-8d72-4e2a-8997-45561a87s510
Info: VNC has been successfully enabled for instance: blossom2
Info: VNC URL: https://vnc.lon1.civo.com/34e0e710-8d72-4e2a-8997-45561a87s510/vnc.html?path=34e0e710-8d72-4e2a-8997-45561a87s510/websockify
Info: We're preparing the VNC Console Access. This may take a while...
Info: New attempt to reach the VNC Console URI...
Info: New attempt to reach the VNC Console URI...
Info: Opening VNC in your default browser...
Info: VNC session is now active. You can access your instance's graphical interface.