Apply build variables to HipChat configuration

In particular this allows specifying the auth token in configuration as
${SECRET_HIPCHAT_TOKEN}, and defining it somewhere else, like with
Jenkins' global password support, so that it isn't stored in the job's
config.xml.

To test this I added a global password named HIPCHAT_AUTH_TOKEN in
global configuration and referenced it as ${HIPCHAT_AUTH_TOKEN} from the
global HipChat configuration. Then I enabled global passwords as
environment variables and HipChat notifications in a build notifications
in a job, built the job, and verified that HipChat was notified.