Cluster page doesn't allow to specify the environment pattern

When having multiple clusters, they are intended to be associated to specific environments using patterns.

The current implementation doesn't include a way to define which pattern is used (it is always * like in CE version), and also there is no check on the duplicates that should be forbidden also at creation level.

The description of what was intended is at https://gitlab.com/gitlab-org/gitlab-ee/issues/3734#note_49128230 and https://gitlab.com/gitlab-org/gitlab-ee/issues/3734#note_49136167.

At the moment, the feature is completely broken because:

• you cannot specify a pattern
• yon cannot edit clusters (error on save because environment pattern conflicts)
• deployments cannot be done on different clusters