Add the plugin repo to the apiman docker image

[KurtStam]

All the work referenced in https://issues.jboss.org/browse/APIMAN-1179 should be done on ipaas:

Add dependency to the plugins:

<dependency>
    <groupId>io.apiman.plugins</groupId>
    <artifactId>apiman-plugins-repository</artifactId>
    <version>1.2.7.Beta1</version>
    <classifier>repo</classifier>
    <type>zip</type>
</dependency>

Unzip into the docker image and configure like

# Here is how we reference the local plugin repository in AMG standalone:
apiman.plugins.repositories=file:///${jboss.home.dir}/apiman/plugins/repository/

# And here is how we reference the local plugin *registry* (the json file):
apiman-manager.plugins.registries=file:///${jboss.server.config.dir}/amg-plugin-registry.json

# The manager uses the apiman.plugins.repositories property directly, but the gateway
# does not.  Instead see below for the gateway config:

# When configuring the plugin registry (gateway config) we do this:
apiman-gateway.plugin-registry=io.apiman.gateway.engine.impl.DefaultPluginRegistry
apiman-gateway.plugin-registry.pluginsDir=${jboss.server.data.dir}/apiman/plugins
apiman-gateway.plugin-registry.pluginRepositories=${apiman.plugins.repositories}