Audit plugin: Add a possibility to turn off log rotation

Based on the testing comments:

well it's easy thing to do. The initial idea was if the server_audit_file_rotations is 0,
that means no rotations at all.
The i decided not to do that as one has to limit the size of the log file somehow anyway.
And one can virtually turn off the rotation by setting a huge server_audit_file_rotate_size.
Elena, what is your opinion about that?

I absolutely think that it should be done, especially if it's an easy thing to do. Doing it via setting server_audit_file_rotations to 0 is fine with me, it's intuitive enough, I actually tried it the first thing after I saw that there was no boolean variable for rotation, only to find out that it didn't work, the value was adjusted to 1.

Doing it via server_audit_file_rotate_size, on the other hand, is dirty and is only acceptable as a temporary workaround.