Refactor LVM plugins

Presently holland has two LVM plugins mysql-lvm (file-level backups) and mysqldump-lvm (logical mysql backups). These are tied directly to MySQL. These are fairly messy and have a dated design. I think it makes sense to refactor these into three plugins:

1) generic LVM support
2) mysql-lvm (inherits from generic lvm)
3) mysqldump-lvm (inherit from mysql-lvm)

Where the mysql plugins just implement the correct quiesce and archiving methods. This is probably a good stepping stone towards a generic snapshot facility as well.