Add support for optional__args and prescript to mysqldump backup provider.

[markleary]

The mysqldump class already has the optional_args and prescript parameters but they were never implemented in the script template.

[JCotton1123]

We're in need of this feature so that we can pass the --master-data=2 option to mysqldump, and use our backups for setting up slaves. What needs to happen to get this merged?

[david22swan]

@markleary My apologies for the wait. In order to merge this PR into the master some manner of test coverage would have to be added to ensure that no problems arise now or in the future.

[david22swan]

@markleary Is there any movement on this PR?

[markleary]

@david22swan I'll try to take a look within the next few days and add tests.

[mjuszczak]

@markleary Just wanted to ping on this. Is there any movement? Do you need any help re: tests? Thank you!

[markleary]

Sorry for the delay, what would you be looking for here as far as tests? Would a prescript test similar to the existing postscript one suffice? @mjuszczak if you have time and want to add the test feel free!