Removing absolute paths to executables

Adds includepath value to cmd_standby.ini.sample and cmd_archiver.ini.sample

* Used to locate executables 

Implements CMDWorker.get_bin_paths_func() in cmd_worker.py

 * Gets paths of all needed external executables using includepath value in given config file. If running as archiver, will also use $PATH variable. 

Implements CMDStandby.get_remote_psql() in cmd_standby

* Uses includepath value in config file and SSH to locate psql executable on master. Removes now-unnecessary config values from code and sample configs (rsync_bin, ssh, pg_ctl, pg_archivecleanup, pg_standby, r_psql...)

Updates documentation accordingly