Derive the CMS hst host configuration and mount from the hst:cmslocation property instead of having to configure it

It is cumbersome to have to configure the correct host configs for cms in hst to support the REST calls for the template composer.

Instead, we can better derive this structure from the newly introduced hst property.

Namely:

hst:cmslocation

This property is needed for cms sso redirect but also for things like SURF & EDIT

Now, this property contains the information for the HST how to create the CMS HOST:

Following rules:

1) If the cms host is a host that does not yet exist in the hst model, we'll add this host and add a hst:root: This root configures the mount with the properties below
2) If the cms host already exists, we add a mount called _rp

What we also need to expose through the ContextualizableMounts is then:

1) What is the contextpath of the mount (/site by default)
2) What is the REST cms Mount (either / or /_rp)

<sv:node sv:name="_rp OR hst:root">
<sv:property sv:name="jcr:primaryType" sv:type="Name">
<sv:value>hst:mount</sv:value>
</sv:property>
<sv:property sv:name="jcr:uuid" sv:type="String">
<sv:value>6db1c591-92ed-47f1-9e3c-ffd2131fa4af</sv:value>
</sv:property>
<sv:property sv:name="hst:ismapped" sv:type="Boolean">
<sv:value>false</sv:value>
</sv:property>
<sv:property sv:name="hst:namedpipeline" sv:type="String">
<sv:value>ComposerPipeline</sv:value>
</sv:property>
<sv:property sv:name="hst:type" sv:type="String">
<sv:value>composer</sv:value>
</sv:property>
</sv:node>