lookup pillar

Generally, this module should be configured as the final ext_pillar, if other
ext_pillars are used.

A pillar value matching the pattern ``${...}`` will trigger this module to perform
a lookup. A lookup may be a pillar value (e.g., ``${other_value}``) or a call to
an execution module (``${cmd.run('echo "foo"')}``). Note that module functions are
executed on the master. Nested functions are supported, as is the passing of
a pillar value to a function. E.g.: ``${cmd.run(command)}``