Adds a note about prerequisites on vCenter
Adds a note about prerequisites on vCenter

We should mention that before
performing any instructions on enabling vCenter
in Fuel, user should make sure that:
- vCenter is accessible
- the login and the password are correct.

This note will be added to the
corresponding section
of the User Guide.

Change-Id: Idea1c9776259cde789abaa0f31228c64b9db4fbc
Related-Bug: 1455116

stackforge/fuel-docs,ogelbukh/fuel-docs,stackforge/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs