Support for variables in ports

[rbarriuso]

Currently the ports section in the descriptor file only accepts numbers (because of schema validation). Nevertheless the generated Dockerfile accepts env variables when exposing ports, for example: EXPOSE ${KIE_ROUTER_PORT}.

When adding a variable in dogen to the port section the following error is displayed.

2017-07-14 11:49:25,894 dogen        ERROR    <SchemaError: error code 2: Schema validation failed:
 - Value '${KIE_ROUTER_PORT}' is not of type 'int'. Path: '/ports/0/value'.: Path: '/'>

We should evaluate the convenience of supporting env variables as port values.

Moreover, currently many many ports are exposed with a fixed value, though they're configurable via variable. For example, here the default port of Jolokia (8778) is exposed, though it can be configured using the AB_JOLOKIA_PORT variable.
Therefore, if the AB_JOLOKIA_PORT env variable is set to a value other than 8778, the service will listen in that other port but the exposed port to Docker would still be 8778.