Currently instances of PostgresNode find their Postgres executables in
the PATH of the caller. This modification allows for instances that know
the installation path they are supposed to use, and the module adjusts
the environment of methods that call Postgres executables appropriately.

This facility is activated by passing the installation path to the
constructor:

  my $node = PostgresNode->get_new_node('mynode',
     installation_path => '/path/to/installation');

This makes a number of things substantially easier, including

. testing third party modules
. testing different versions of postgres together
. testing different builds of postgres together

Discussion: https://postgr.es/m/a94c74f9-6b71-1957-7973-a734ea3cbef1@dunslane.net

Reviewed-By:  Alvaro Herrera, Michael Paquier, Dagfinn Ilmari Mannsåker