Improve 'abaco init' to report a list of project types by tenant

[mwvaughn]

End users do not know what project types are available for a given tenant. Given that it is easy to contribute new ones to the CLI via PR, it is expected that diversification in type, configuration, and naming will happen quickly between tenants.

It should be possible to iterate over the tenant-specific template directories and return at least the template directory name if not some little bit of metadata included in each directory.

For instance, for the sd2e tenant, the output from abaco init -h might look like so, where the * next to reactor-python indicates it is the default:

Usage: abaco init [OPTION]... [IMAGE]

Initializes an Abaco actor project in a new directory.

The default value for -O can be changed by setting the
DOCKER_HUB_ORG environment variable (e.g. DOCKER_HUB_ORG=tacc).

Options:
  -h    show help message
  -n    project name (e.g. my-actor-name)
  -d    project description
  -l    project type (bionic-python | disco-python | reactor-python* | shell)
  -O    registry username/organization (sd2eadm)
  -B    base path (current directory)