all: Pin Ubuntu kernel to 5.0.0-37 for Bionic

all: Pin Ubuntu kernel to 5.0.0-37 for Bionic

Ubuntu kernel meta packages are all broken on at least one platform
architecture, so pin the kernel version to 5.0.0-37, which is known to
be stable.

Make the kernel version configurable via a new enviroment variable,
MCP_KERNEL_VER in globals.sh. If not defined, the ga-18.04 kernel is
left unchanged (based on upstream kernel 4.15), except for baremetal
nodes providioned by MaaS which currently use the HWE kernel (based on
5.3 in Bionic).

Change-Id: I648d09b22f6080efd2bce26b6a06fecc3f6b4599
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>