Fix Environment list Vue keys in for loop

The following discussion from gitlab-ce!21018 should be addressed:

• @mikegreiling started a discussion: (+3 comments)

  This is a change made in the EE-side due to the deploy boards requiring more specific key names. I see no reason not to backport it here to reduce conflicts.

Currently the environment list uses the array index value as a key for the Vue for loop when we could use the environment ID or name (which should be unique) to do this more efficiently.

/cc @jivanvl