maniacs-setup fails when downloading virtualization images if ~/.maas-cert-server doesn't exist

[Rod Smith]

When running "maniacs-setup --download-virtualization-images" recently, the script asked the usual questions about what to download and then produced a series of errors like this:

wget: /home/rodsmith/.maas-cert-server/cloudimg-root-dl-xenial-amd64.log: No such file or directory

Creating the ~/.maas-cert-server directory caused it to succeed on the next run.

It looks like the script creates ~/.maas-cert-server (aka $MCS_DATA) in some functions that require it, but not in retrieve_virtualization_image(). Thus, either a call to create that directory should be added to that function or the directory should be created in some globally-called function rather than in each function that might use the directory.