ctdb-tests: Add new test functions for running commands on nodes

ctdb-tests: Add new test functions for running commands on nodes

* ctdb_onnode()
* testprog_onnode()
* function_onnode()

These encapsulate familiar patterns found when running
try_command_on_node().  The new function names are more concise and
encourage more readable tests.  Test writers can do less thinking
about the subtleties of running different types of commands on nodes.
For example, these functions ensure that $CTDB and $VALGRIND are used
in the correct contexts.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>