add text variable input from variables

What does this MR do?

Adds text inputs for each custom variable

Issue #214503 (closed)

Demo

https://gitlab-review-jivanvl-ad-oucqjp.gitlab-review.app/gitlab-qa-sandbox-group/qa-test-2020-05-05-23-42-00-a38483f4bc32da84/autodevops-project-24cfa0746b8a228a/-/environments/1/metrics?pod=namese

Screenshots

Variables section	Demo video
	Screen_Recording_2020-05-07_at_13.59.24

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides