fix guice injection error if HttpServer is disabled

if http.enabled is set to false a node will be started
    without HttpServer bound and guice will throw error
    because it is used in the NodePortExpression
added graceful_stop cluster settings
documentation for zero downtime cluster upgrade
documentation for graceful_stop cluster settings
Merge pull request #1111 from crate/h/stop-settings

gradeful_stop settings
Merge pull request #1113 from crate/h/stop-settings-docs

graceful stop docs
implemented rounting.allocation.enable setting
test fixup
add SQLService that handles disabling of SQL transports