Code review comment for ~raharper/cloud-init:fix/debian-config-yaml-spaces

OK, I've combined Chad's clean-up and then emitted that as the value, like this:

LISTVAL="$(sed -e "s/\s//g" -e "/^$key:/"'!'d \
               -e "s/$key:\[//;s/]//;s/,/, /g" "$file")"
RET="$key: [ ${LISTVAL} ]"

In action looks like this:

# Edge-case, single entry
root@x1:~# cat /etc/cloud/cloud.cfg.d/90_dpkg.cfg
# to update this file, run dpkg-reconfigure cloud-init
datasource_list: [ ConfigDrive ]
root@x1:~# ./test3.sh /etc/cloud/cloud.cfg.d/90_dpkg.cfg
datasource_list: [ ConfigDrive ]

# Edge-case, empty
root@x1:~# cat /etc/cloud/cloud.cfg.d/90_dpkg.cfg.empty
# to update this file, run dpkg-reconfigure cloud-init
datasource_list: []
root@x1:~# ./test3.sh /etc/cloud/cloud.cfg.d/90_dpkg.cfg.empty
datasource_list: [ ]

# the bug case
root@x1:~# cat /etc/cloud/cloud.cfg.d/90_dpkg.cfg.bad
# to update this file, run dpkg-reconfigure cloud-init
datasource_list: [ConfigDrive, None]
root@x1:~# ./test3.sh /etc/cloud/cloud.cfg.d/90_dpkg.cfg.bad
datasource_list: [ ConfigDrive, None ]

# the default case
root@x1:~# cat /etc/cloud/cloud.cfg.d/90_dpkg.cfg.new
s file, run dpkg-reconfigure cloud-init
datasource_list: [ NoCloud, ConfigDrive, OpenNebula, DigitalOcean, Azure, AltCloud, OVF, MAAS, GCE, OpenStack, CloudSigma, SmartOS, Bigstep, Scaleway, AliYun, Ec2, CloudStack, Hetzner, IBMCloud, Oracle, Exoscale, None]
root@x1:~# ./test3.sh /etc/cloud/cloud.cfg.d/90_dpkg.cfg.new
datasource_list: [ NoCloud, ConfigDrive, OpenNebula, DigitalOcean, Azure, AltCloud, OVF, MAAS, GCE, OpenStack, CloudSigma, SmartOS, Bigstep, Scaleway, AliYun, Ec2, CloudStack, Hetzner, IBMCloud, Oracle, Exoscale, None ]