Limit parallel to 100

This prevents some of the abusive behaviors, of someone putting 100000 and creating out of memory condition easily

doc/ci/yaml/README.md

@@ -1510,7 +1510,7 @@ Possible values for `when` are:
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22631) in GitLab 11.5.
 
 `parallel` allows you to configure how many instances of a job to run in
-parallel. This value has to be greater than or equal to two (2).
+parallel. This value has to be greater than or equal to two (2) and less or equal than 50.
 
 This creates N instances of the same job that run in parallel. They're named
 sequentially from `job_name 1/N` to `job_name N/N`.

lib/gitlab/ci/config/entry/job.rb

@@ -27,7 +27,8 @@ module Gitlab
               validates :tags, array_of_strings: true
               validates :allow_failure, boolean: true
               validates :parallel, numericality: { only_integer: true,
-                                                   greater_than_or_equal_to: 2 }
+                                                   greater_than_or_equal_to: 2,
+                                                   less_than_or_equal_to: 50 }
               validates :when,
                 inclusion: { in: %w[on_success on_failure always manual delayed],
                              message: 'should be on_success, on_failure, ' \

spec/lib/gitlab/ci/config/entry/job_spec.rb

@@ -118,6 +118,16 @@ describe Gitlab::Ci::Config::Entry::Job do
           end
         end
 
+        context 'when it is bigger than 50' do
+          let(:config) { { parallel: 51 } }
+
+          it 'returns error about value too high' do
+            expect(entry).not_to be_valid
+            expect(entry.errors)
+              .to include 'job parallel must be less than or equal to 50'
+          end
+        end
+
         context 'when it is not an integer' do
           let(:config) { { parallel: 1.5 } }