fix(qemu_template): Replace getopts with simple pattern matching.

The use of getopts was leading to conflicts between this script's short
options and qemu's long options. For example -serial was getting
interpreted as -s -- erial which is not very helpful.
feat(qemu_template): Provide SSH keys via config drive

Moving away from our old behavior of loading 'authorized_keys' on virtfs
volumes. Still using virtfs to avoid depending on mkisofs or similar.
feat(qemu_template): Add support for user-provided configs.

A custom cloud config can be provided directly via -u or as an existing
config drive image via -c.
Merge pull request #214 from marineam/units

Config drive for QEMU