Optional ESP bootloader installation

[vjr]

Distribution (run cat /etc/os-release):

See issue elementary/installer#662
and PR elementary/installer#675

Related Application and/or Package Version (run apt policy $PACKAGE NAME):

elementary OS installer.

Issue/Bug Description:

Related to the linked issue and PR this is a request to make bootloader installation optional for use cases where people are dual/multi booting different OS and do not want a distinst based installation overwrite their preferred bootloader setup.

Steps to reproduce (if you know):

Currently running the elementary OS (v8 early access) installer with UI allows to skip bootloader installation but distinst reports error (paraphrasing) "EFI bootloader not specified".

Expected behavior:

Installation should success without bootloader if the user so chooses.

Other Notes:
NA