add post-send script option

We need post-send script option to configure presendScript in the
generated configure xml.
When setting post-send as below:
   postsend-script: $DEFAULT_POSTSEND_SCRIPT
We could see below informaton in the generated configure xml.
<hudson.plugins.emailext.ExtendedEmailPublisher plugin="email-ext@2.42">
 <postsendScript>$DEFAULT_POSTSEND_SCRIPT</postsendScript>
</hudson.plugins.emailext.ExtendedEmailPublisher>

Change-Id: Ic215b2f3a9b5e7361a2cc1772226382da341ecd1
Signed-off-by: Liu Changcheng <changcheng.liu@intel.com>
Signed-off-by: Jerry Liu <primerlink@gmail.com>

tests/publishers/fixtures/email-ext001.xml

@@ -164,6 +164,7 @@
       <defaultContent>The build has finished</defaultContent>
       <attachmentsPattern>*/foo*.log</attachmentsPattern>
       <presendScript>cancel=true</presendScript>
+      <postsendScript>cancel=true</postsendScript>
       <attachBuildLog>false</attachBuildLog>
       <compressBuildLog>false</compressBuildLog>
       <saveOutput>true</saveOutput>

tests/publishers/fixtures/email-ext001.yaml

@@ -24,6 +24,7 @@ publishers:
       pre-build: true
       matrix-trigger: only-configurations
       presend-script: "cancel=true"
+      postsend-script: "cancel=true"
       save-output: true
       send-to:
         - developers

tests/publishers/fixtures/email-ext002.xml

@@ -21,6 +21,7 @@
       <defaultContent>The build has finished</defaultContent>
       <attachmentsPattern>*/foo*.log</attachmentsPattern>
       <presendScript/>
+      <postsendScript/>
       <attachBuildLog>true</attachBuildLog>
       <compressBuildLog>true</compressBuildLog>
       <saveOutput>false</saveOutput>

tests/yamlparser/fixtures/template_override_project_level_defaults.xml

@@ -32,6 +32,7 @@
       <defaultContent>$DEFAULT_CONTENT</defaultContent>
       <attachmentsPattern/>
       <presendScript/>
+      <postsendScript/>
       <attachBuildLog>false</attachBuildLog>
       <compressBuildLog>false</compressBuildLog>
       <saveOutput>false</saveOutput>