ProcessMaker-MA "Process Variables (fixes & behat)"

- Se han agregado validaciones faltantes
- Se han implementado los siguientes features (behat):
    GET /api/1.0/{workspace}/project/{prj_uid}/variables
    GET /api/1.0/{workspace}/project/{prj_uid}/grid/variables
    GET /api/1.0/{workspace}/project/{prj_uid}/grid/{grid_uid}/variables

features/backend/process_variables/basic_sequence.feature

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+@ProcessMakerMichelangelo @RestAPI
+Feature: Process Variables
+    Requirements:
+        a workspace with the process 14414793652a5d718b65590036026581 ("Sample Project #1") already loaded
+        there are three activities in the process
+
+    Background:
+        Given that I have a valid access_token
+
+    #GET /api/1.0/{workspace}/project/{prj_uid}/variables
+    #    Get all variables of a Process
+    Scenario Outline: Get all variables of a Process
+        And I request "project/14414793652a5d718b65590036026581/variables"
+        And the content type is "application/json"
+        Then the response status code should be 200
+        And the response charset is "UTF-8"
+        And the type is "array"
+        And the "var_name" property in row <i> equals "<var_name>"
+
+        Examples:
+        | i | var_name |
+        | 0 | SYS_LANG |
+        | 1 | SYS_SKIN |
+        | 2 | SYS_SYS  |
+
+    #GET /api/1.0/{workspace}/project/{prj_uid}/grid/variables
+    #    Get grid variables of a Process
+    Scenario: Get grid variables of a Process
+        Given I request "project/14414793652a5d718b65590036026581/grid/variables"
+        And the content type is "application/json"
+        Then the response status code should be 200
+        And the response charset is "UTF-8"
+        And the type is "array"
+        And the json data is an empty array
+
+    #GET /api/1.0/{workspace}/project/{prj_uid}/grid/{grid_uid}/variables
+    #    Get all variables of a Grid
+    Scenario Outline: Get all variables of a Grid
+        Given I request "project/14414793652a5d718b65590036026581/grid/00000000000000000000000000000000/variables"
+        And the content type is "application/json"
+        Then the response status code should be <status_code>
+        And the response charset is "UTF-8"
+        And the type is "object"
+        And the response status message should have the following text "<status_message>"
+
+        Examples:
+        | status_code | status_message |
+        | 400         | grid_uid       |
+