Re-use ssh connection

- Previously only re-using if no flags were supplied; now re-use also
  if it's just that stdout/stderr have custom capturing.
  Important for SshSensorAdapter.
Add `MySqlNode.DATA_DIR` config key for specifying the location of the DB files
Specify correct --datadir option.
Test override of MySQL data location in integration test.
Merge pull request #443 from aledsage/fix/reuse-ssh-connection

Re-use ssh connection
Merge pull request #432 from alasdairhodge/mysql-data-location-config

Permit configuration of MySQL data directory