[JEP-305] Make CasC incrementals-ready

Used `mvn incrementals:incrementalify`.

See https://github.com/jenkinsci/incrementals-tools/blob/master/README.md

.mvn/extensions.xml

@@ -0,0 +1,7 @@
+<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 http://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
+  <extension>
+    <groupId>io.jenkins.tools.incrementals</groupId>
+    <artifactId>git-changelist-maven-extension</artifactId>
+    <version>1.0-beta-3</version>
+  </extension>
+</extensions>

.mvn/maven.config

@@ -0,0 +1,2 @@
+-Pconsume-incrementals
+-Pmight-produce-incrementals

pom.xml

@@ -10,10 +10,12 @@
   </parent>
   <groupId>io.jenkins</groupId>
   <artifactId>configuration-as-code</artifactId>
-  <version>0.6-alpha-SNAPSHOT</version>
+  <version>${revision}${changelist}</version>
   <packaging>hpi</packaging>
 
   <properties>
+        <revision>0.6-alpha</revision>
+        <changelist>-SNAPSHOT</changelist>
     <jenkins.version>2.60.3</jenkins.version>
     <java.level>8</java.level>
   </properties>
@@ -41,7 +43,7 @@
     <connection>scm:git:git://github.com/jenkinsci/${project.artifactId}-plugin.git</connection>
     <developerConnection>scm:git:git@github.com:jenkinsci/${project.artifactId}-plugin.git</developerConnection>
     <url>https://github.com/jenkinsci/${project.artifactId}-plugin</url>
-    <tag>HEAD</tag>
+    <tag>${scmTag}</tag>
   </scm>
 
   <repositories>