filesystem/config: add esp flag to a UEFI boot partition

[jouvin]

As stated in quattor/template-library-core#152, a UEFI boot partition should have both boot and esp flags to be label-type independent. Template library should be updated to set these flags instead of the GPT-specific bios_grub after quattor/template-library-core#153 is merged.

[jrha]

As noted by @stdweird in quattor/template-library-core#153 parted in EL7.3 doesn't support esp, so unless we can ensure it works, we should avoid it.