Merge pull request #1509 from jedwards4b/allowed_schema_skip

Fix env_mach_specific validation

Allow the user the option of skipping the xmllint validation of cime xml files.

Test suite: scripts_regression_tests.py
Test baseline:
Test namelist changes:
Test status: bit for bit

Addresses #1505

User interface changes?: Adds --skip-xml-validation flag to all user interface scripts

Code review:

config/xml_schemas/env_mach_specific.xsd

@@ -23,9 +23,9 @@
       <xs:sequence>
         <xs:element ref="header"/>
         <xs:element ref="module_system"/>
-        <xs:element maxOccurs="unbounded" ref="environment_variables"/>
-        <xs:element maxOccurs="unbounded" ref="mpirun"/>
-        <xs:element maxOccurs="unbounded" ref="entry"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="environment_variables"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="mpirun"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="entry"/>
       </xs:sequence>
       <xs:attribute ref="id" use="required"/>
       <xs:attribute ref="version" use="required"/>
@@ -35,9 +35,9 @@
   <xs:element name="module_system">
     <xs:complexType>
       <xs:sequence>
-        <xs:element maxOccurs="unbounded" ref="init_path"/>
-        <xs:element maxOccurs="unbounded" ref="cmd_path"/>
-        <xs:element maxOccurs="unbounded" ref="modules"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="init_path"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="cmd_path"/>
+        <xs:element minOccurs="0" maxOccurs="unbounded" ref="modules"/>
       </xs:sequence>
       <xs:attribute name="type" use="required" type="xs:NCName"/>
     </xs:complexType>