Add a /properties endpoint to the REST API

[p013570]

This should allow a predefined set of properties/configuration to be exposed.

GET /properties should return a map of all exposed property key values.
GET /properties/{name} should return the value of the property with the given name.

The set of exposed properties should be configured in rest.properties. E.g

gaffer.properties=gaffer.properties.app-description,some-other-property
gaffer.properties.app-description="This is a description about the this application"
gaffer.properties.some-other-property="This is some other property"
gaffer.properties.some-hidden-property="This is some other hidden property"

So /properties would return:

{
"gaffer.properties.app-description": "This is a description about the this application",
"gaffer.properties.some-other-property": "This is some other property",
}

/properties/gaffer.properties.app-description would return:

"This is a description about the this application"

/properties/gaffer.properties.some-hidden-property would throw some error saying something like "gaffer.properties.some-hidden-property" is not a recognised property.

The system-description property could then be used in issue #1419.

[p013570]

Merged into develop.