Add RHEL support to snp.sh

1. Changed AMDSEV URL and AMDSEV branch for AMDSEV build with RHEL fixes

2. rhel_install_dependencies for rhel library package manager dependencies for AMDSEV branch.
      requires subscription manager credential for installing RedHat
libraries

3. set_grub_default_snp() using grubby tool for RHEL

4. Modified save_binary_paths() due to the differences in the location of guest kernel file path for ubuntu and rhel and differences in the boot menu for initial ram disk images(initd.img-<kernel-version> for ubuntu and initramfs-<kernel-version> for rhel)

Signed-off-by: Harika <lnittala@amd.com>