Update the lvlab hosts --append command to verify before appending

[memblin]

As an lvlab user the lvlab hosts --append command can sometimes add duplicate content to the /etc/hosts file. We should update the feature to do some validation before appending.

Potential validations:

• Does this IP already exist in /etc/hosts
• Does this hostname already exists in /etc/hosts
• Does this fqdn already exist in the /etc/hosts

If any of the items for a machine already exists, skip that entry and output that it was skipped and why.
Add non-conflicting entries as appropriate.

AC

• The lvlab hosts --append command no longer adds duplicate values to the /etc/hosts file
• The output from a conflicting entry is easy to understand
• When writing machine /etc/hosts the local machine name is excluded allowing 127.0.0.1 and ::1 to work as expected