Modify HDP stack requirements for 6.6.2

With the introduction of SOLR-7.3.1, we now have two versions of the
SOLR service that (according to their metainfo.xml and mpack.json
specs) are meant for HDP-2.6.  This commit modifies the service
metadata so that SOLR 5.5.5 is used on HDP 2.4, SOLR 6.6.2 is used on
HDP 2.5, and SOLR 7.3.1 is used on HDP 2.6.

Author: Jason Gerlowski

src/main/mpack/extensions/SOLR/6.6.2/metainfo.xml

@@ -17,7 +17,7 @@
     <stacks>
         <stack>
             <name>HDP</name>
-            <version>2.6</version><!-- TODO test only it should be 3.0 (mpack) -->
+            <version>2.5</version><!-- TODO test only it should be 3.0 (mpack) -->
         </stack>
     </stacks>
 </metainfo>

src/main/mpack/mpack.json

@@ -45,7 +45,7 @@
           "applicable_stacks": [
             {
               "stack_name": "HDP",
-              "stack_version": "2.6"
+              "stack_version": "2.5"
             }
           ]
         },
@@ -55,7 +55,7 @@
           "applicable_stacks": [
             {
               "stack_name": "HDP",
-              "stack_version": "2.6.4"
+              "stack_version": "2.6"
             }
           ]
         }