Allow more customization for CDash reporter

Add new command line arguments to `spack install` that allow users
to set the build name, site name, and track in their CDash report.

share/spack/templates/reports/cdash/Site.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<Site BuildName="{{ install_command }}"
+<Site BuildName="{{ buildname }}"
       BuildStamp="{{ buildstamp }}"
-      Name="{{ hostname }}"
+      Name="{{ site }}"
       OSName="{{ osname }}"
 >
 

share/spack/templates/reports/cdash/Update.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Update>
-  <Site>{{ hostname }}</Site>
-  <BuildName>{{ install_command }}</BuildName>
+  <Site>{{ site }}</Site>
+  <BuildName>{{ buildname }}</BuildName>
   <BuildStamp>{{ buildstamp }}</BuildStamp>
   <StartTime>{{ starttime }}</StartTime>
   <EndTime>{{ endtime }}</EndTime>