Allow users to share the template/keys config element between templates

[looztra]

For the moment, when you have more than one templates you have to repeat the "keys" section for all of them

            <plugin>
                <groupId>com.github.nodevops</groupId>
                <artifactId>confd-maven-plugin</artifactId>
                <executions>
                    <execution>
                        <!-- common step shared by all profiles that prepare the config file for further usage /-->
                        <id>prepare</id>
                        <goals>
                            <goal>prepare</goal>
                        </goals>
                        <configuration>
                            <forceDestToLocalFileSystemType>true</forceDestToLocalFileSystemType>
                            <templates>
                                <template>
                                    <id>application.properties</id>
                                    <src>src/main/confd/templates/application.properties.tmpl</src>
                                    <dest>${project.basedir}/target/generated-configuration/application.properties</dest>
                                    <keys>
                                        <value>/web</value>
                                        <value>/shared</value>
                                        <value>/runtime</value>
                                    </keys>
                                </template>
                                <template>
                                    <id>hazelcast-client.xml</id>
                                    <src>src/main/confd/templates/hazelcast-client.xml.tmpl</src>
                                    <dest>${project.basedir}/target/classes/hazelcast-client.xml</dest>
                                    <keys>
                                        <value>/web</value>
                                        <value>/shared</value>
                                        <value>/runtime</value>
                                    </keys>
                                </template>
                            </templates>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

It may be nice to be able to declare the keys once (associated with an id?) and reference them in the template config element