Foreman Maintain

Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1894149

Description of problem:
Upgrading from 6.7 with /etc/foreman-installer/custom-hiera.yaml :

~_{~~~~~~~~~~~~~}~
s YAML file lets you set your own custom configuration in Hiera for the <=============================================

1. installer puppet modules that might not be exposed to users directly through
2. installer arguments.
   ~_{~~~~~~~~~~~~~}~

Version-Release number of selected component (if applicable):
satellite-6.8.0-0.7.beta.el7sat.noarch
satellite-installer-6.8.0.8-1.el7sat.noarch

How reproducible:
100%

Steps to Reproduce:
1. Have syntax issue in Sat 6.7 with /etc/foreman-installer/custom-hiera.yaml :
~_~~~~~~~~~~~~
s YAML file lets you set your own custom configuration in Hiera for the <=============================================

1. installer puppet modules that might not be exposed to users directly through
2. installer arguments.

~_~~~~~~~~~~~~

2. Run upgrade to 6.8

Actual results:
2. is supposed to fail with below misguiding error to another Bug

~_~~~~~~~~~~~~~
Check if checkpoint_segments configuration exists on the system: [FAIL]
undefined method `key?' for #<String:0x0000000139c960>

~_{~~~~~~~~~~~~~}~

Expected results:
1.) Foreman-maintain command should check for syntax issue in /etc/foreman-installer/custom-hiera.yaml should show error related to syntax and not to any other misguiding error

Additional info: