ha-cluster: Make create-new-cluster to ask less questions

This commit modifies lizardfs-cluster-manager create-new-cluster to ask
only the minimum required set of question to configure the master
server, i.e.:

* if there there no files which are present in default installations,
  ask if a default installation should be created
* otherwise require users to choose a valid master server's
  configuration file.

It also adds some explanations to all the questions. This should make it
easier for a new user to answer them.

Change-Id: Ice96ce1ebca42f7b6a9bc68a9eee1a2a05cc8f92