Make Vault Secret auth method path configurable

Currently Vault Secrets are using partially hardcoded configuration
for the authentication method. While the Vault server URL and the
role name for JWT authentication method can be configured with the
variables, the authentication method path is hardoced to `jwt`.
This may limit the usability of our solution.

This change makes this value configurable with the `VAULT_AUTH_PATH`
variable (similar to how `VAULT_SERVER_URL` and `VAULT_AUTH_ROLE`
are being used already) and ensures that in case when the variable
is not defined by the user, it will fall-back to the `jwt` value
that we have hardcoded now.

ee/app/models/ee/ci/build.rb

@@ -148,8 +148,8 @@ module EE
         variable_value('VAULT_SERVER_URL').present?
       end
 
-      def variable_value(key)
-        variables_hash[key]
+      def variable_value(key, default = nil)
+        variables_hash.fetch(key, default)
       end
 
       private

ee/app/presenters/ee/ci/build_runner_presenter.rb

@@ -19,7 +19,7 @@ module EE
           'url' => variable_value('VAULT_SERVER_URL'),
           'auth' => {
             'name' => 'jwt',
-            'path' => 'jwt',
+            'path' => variable_value('VAULT_AUTH_PATH', 'jwt'),
             'data' => {
               'jwt' => '${CI_JOB_JWT}',
               'role' => variable_value('VAULT_AUTH_ROLE')

ee/spec/presenters/ci/build_runner_presenter_spec.rb

@@ -63,6 +63,24 @@ RSpec.describe Ci::BuildRunnerPresenter do
           end
         end
       end
+
+      context 'Vault auth path' do
+        let(:vault_auth) { presenter.secrets_configuration.dig('DATABASE_PASSWORD', 'vault', 'server', 'auth') }
+
+        context 'VAULT_AUTH_PATH CI variable is present' do
+          it 'contains user defined auth path' do
+            create(:ci_variable, project: ci_build.project, key: 'VAULT_AUTH_PATH', value: 'custom/path')
+
+            expect(vault_auth.fetch('path')).to eq('custom/path')
+          end
+        end
+
+        context 'VAULT_AUTH_PATH CI variable is not present' do
+          it 'contains the default auth path' do
+            expect(vault_auth.fetch('path')).to eq('jwt')
+          end
+        end
+      end
     end
   end
 end