Check Env for exist before validating assignment

If we try to assign some role to some nodes with invalid cluster id:

    $ fuel --env 99 node set --role controller --node 1 2 3

we will get an internal server error (500). Since now, it's fixed.

Change-Id: I9bea9a498295b5db2114874c3502425d7dc94b46
Closes-Bug: #1317703
Merge "Check Env for exist before validating assignment"