Adding support for maven-metadata-plugin

Added a method called maven_metadata_param to deal with maven metadata
(https://wiki.jenkins-ci.org/display/JENKINS/Maven+Metadata+Plugin)
plus addition of relevant tests

Change-Id: Id04b33653cc90f69dd1588286352b9d19e852ece

tests/parameters/fixtures/maven-metadata-param001.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="utf-8"?>
+<project>
+  <properties>
+    <hudson.model.ParametersDefinitionProperty>
+      <parameterDefinitions>
+        <eu.markov.jenkins.plugin.mvnmeta.MavenMetadataParameterDefinition>
+          <name>maven metadata param</name>
+          <description/>
+          <repoBaseUrl>http://nexus.example.com</repoBaseUrl>
+          <groupId>com.example</groupId>
+          <artifactId>example</artifactId>
+          <packaging>jar</packaging>
+          <defaultValue>FIRST</defaultValue>
+          <versionFilter>[0-9]+</versionFilter>
+          <sortOrder>ASC</sortOrder>
+          <maxVersions>5</maxVersions>
+          <username>username</username>
+          <password>password</password>
+        </eu.markov.jenkins.plugin.mvnmeta.MavenMetadataParameterDefinition>
+      </parameterDefinitions>
+    </hudson.model.ParametersDefinitionProperty>
+  </properties>
+</project>

tests/parameters/fixtures/maven-metadata-param001.yaml

@@ -0,0 +1,13 @@
+parameters:
+  - maven-metadata:
+      name: 'maven metadata param'
+      repository-base-url: 'http://nexus.example.com'
+      repository-username: 'username'
+      repository-password: 'password'
+      artifact-group-id: 'com.example'
+      artifact-id: 'example'
+      packaging: 'jar'
+      versions-filter: '[0-9]+'
+      default-value: 'FIRST'
+      maximum-versions-to-display: "5"
+      sorting-order: "Ascending"