[Enhancement]: Enable mungectl to set file access permissions on the munge key file

[NucciTheBoss]

mungectl currently just sets the munge.key file owner to user root and group root. We should allow administrators to set the user and group that will own the munge.key. The root user works as a default, but typically administrators will use a dedicated munge user and group to own the key.

We should also enable administrators to set the mode of the munge.key file. Right now we're using the default mode which works, but some may want to set a mode of 0o600 so that only the munge user can read the key.

This is a relatively easy addition since we just need to add some additional flags to the generate and set subcommands that feed into the os package:

mungectl key generate -u/--user -g/--group -m/--mode
mungectl key set -u/--user -g/--group -m/--mode

...

os.Chmod("/path/to/keyfile", ...)
os.Chown("/path/to/keyfile", ..., ...)

We could also add support for configuring a default user, group, and mode so that the administrator isn't required to pass these flags every time using viper.