maas: Switch back to ga-18.04 kernel during deploy

maas: Switch back to ga-18.04 kernel during deploy

hwe-18.04, currently based on 5.3 kernel in Bionic, has issues on both
x86_64 and aarch64 nodes, so use ga-18.04, currently based on 4.15.

If MCP_KERNEL_VER is set (currently pinned to 5.0), the ga-18.04 kernel
is replaced by the specified version after the MaaS commissioning,
initial MaaS deployment.

Change-Id: Ibe8e27217025290c1263f8dca9496b2cde24368c
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 6ecb2f9a9cd8b0c97002a7ff5583690856c11af0)