This is related to elastic#30134. It modifies the start_trial action to require
an acknowledgement parameter in the rest request to actually start the
trial license. There are backwards compatibility issues as prior ES
versions did not support this parameter. To handle this, it is assumed
that a request coming from a node prior to 6.3 is acknowledged. And
attempts to write a non-acknowledged request to a prior to 6.3 node will
throw an exception.

Additionally this PR adds messages about the trial license the user is
generating.