Improved VCP Documentation

[divyenpatel]

• Clarified putting cloud provider configuration for each releases in separate sections.
• Added steps for storing vsphere credentials to secrets.
• Added documentation for SAML token authentication feature.
• Updated configuration steps, and mentioned only master node requires vspehre.conf file with credentials and on the worker node we just need --cloud-provider=vsphere flag.
• Updated feature list in the Prerequisites section for release 1.11.
• Directed user to read about roles and privileges from https://vmware.github.io/vsphere-storage-for-kubernetes/documentation/vcp-roles.html

Issues: https://github.com/vmware/kubernetes/issues/479, https://github.com/vmware/kubernetes/issues/491

@dougm @SandeepPissay @embano1 @BaluDontu Please take a look.

[divyenpatel]

@SandeepPissay I have addressed your review comments.