Add option to allow setting grub menu visibility and timeout

[Drew Freiberger]

During recovery of a CDK cluster on top of vmware, we were unable to catch a grub prompt to alter kernel boot flags, as the cloud image does not stop at a grub menu.

This needs to be configurable via sysconfig charm such that we can set a 30 or 60 second boot timer and make the grub menu visible on cloud images, whether hosted on Openstack, AWS, Google, or VMware.

[JamesLin (jneo8)]

question: About grub-config-flags config

There is `grub-config-flags` in the sysconfig config. Can we use this config to set the timer?

[JamesLin (jneo8)]

Update: I test sysconfig on my local virtual-maas with this command

```
juju config sysconfig update-grub=true grub-config-flags='GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=menu
GRUB_SERIAL_COMMAND="serial --speed=115200"
GRUB_TERMINAL="console serial"
'
```

And deploy on ubuntu(focal)

The `/etc/default/grub.d/90-sysconfig.cfg` will be config as

```
# Juju generated file - do not edit manually

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'

# Evaluate grub-config-flags first, in case it uses $GRUB_CMDLINE_LINUX_DEFAULT.
# However, if grub-config-flags is not specified, ignore that (current behavior.)
# config-flags
GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=menu
GRUB_SERIAL_COMMAND="serial --speed=115200"
GRUB_TERMINAL="console serial"
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT console=ttyS0,115200 console=tty0"
# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console
GRUB_TERMINAL=serial
```

The `GRUB_TERMINAL` will be override by default `GRUB_TERMINAL=serial`(Not sure why two config not merge), which make `console` be disabled and grub can't view on `virt-viewer`

(I tried to comment `GRUB_TERMINAL=serial` manual, and grub showed on `virt-viewer`)

I am not sure which TERMINAL type Openstack, AWS, Google, or VMware need.

But maybe is because `GRUB_TERMINAL`?