RestProxyServicePlugin should lazily check whether sites are available

currently, at first login in cms, directly a call to the hst is done whether the sites are available. This is even done when you dont have a channel manager at all! Thus when you do not include

<dependency>
<groupId>org.onehippo.cms7</groupId>
<artifactId>hippo-addon-channel-manager-frontend</artifactId>
</dependency>

<dependency>
<groupId>org.onehippo.cms7</groupId>
<artifactId>hippo-addon-channel-manager-repository</artifactId>
</dependency>

still a check for sites is done. This is because org/hippoecm/frontend/service/restproxy/RestProxyServicePlugin.java in

public RestProxyServicePlugin(IPluginContext context, IPluginConfig config) {

directly calls checkSiteIsAlive(pingServiceUri);

This also makes the initial login slower than needed. We should only check this afaik during #createRestProxy and #createSecureRestProxy

Note that then before clicking the channel mngr icon, you cannot know whether the channels are available or not