Fix displaying tag from DialogFieldTagControl

[lpichler]

We are able to keep only one value of selected
tag in DialogFieldTagControl, fix me if I am wrong

Test Scenario

• Create Service Dialog With Tag Element
  
• use it in service
• order the service and choose any tag
  
• review the request:

Before:

After:

@miq-bot assign @himdel (not sure who is right person for the review)
@miq-bot add_label bug

[miq-bot]

@lpichler 'himdel (not sure who is right person for the review)' is an invalid assignee, ignoring...

[miq-bot]

Checked commit lpichler@3aa70f1 with ruby 2.2.6, rubocop 0.47.1, and haml-lint 0.20.0
1 file checked, 0 offenses detected
Everything looks fine. ⭐

[eclarizio]

I think the change itself is fine, but since we're modifying the logic anyway, I think there's a lot of value in pulling it out into a helper function that we can then write a spec for.