bootloader: Add an option to override the tuned_initrd path in grub.cfg

[NeilHanlon]

• Detection of the location from /proc/cmdline can be wrong in some cases. This option allows a user to point to the place the tuned initrd actually is as per their grub config.

Thank you in advance! Please let me know if there's anything I missed e.g. tests, etc.. I didn't see any tests for the bootloader plugin yet but I could add one for this option, if necessary.

Best, Neil