Fixes value of validate_id in host form

[rvsia]

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1634794

Description of problem:
When 2 or more hosts are selected and edited, Alert 'Abandon changes' is displayed even when no changes are made by the user.

Steps to Reproduce:

1. Have CFME appliance with added provider with some hosts
2. Go to Compute -> Infrastructure -> Hosts
3. Select 2 or more hosts and click Configuration -> Edit selected items
4. Leave the page without making any changes

Actual results:
Alert asking whether to abandon changes is displayed

Expected results:
If no changes are made, no 'Abandon changes' alert should be displayed

Solution

• validation_id was nil in template and blank in controller
• => changed to null everywhere
• it caused dirty state on the form (because the model and the copy was different)

Before (immediately after loading of the form)

After (notice form buttons)

[rvsia]

@miq-bot add_label hammer/yes, bug, compute/infrastructure,

[miq-bot]

Checked commit rvsia@6e66bc5 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 1 offense detected

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• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby23, which recognizes
warning: 2.3.6-compliant syntax, but you are running 2.3.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.