Refactor templates system to include /tenants service hostname

[mwvaughn]

To support templates for non-TACC Abaco deployments, we will add an additional level to the templates directory corresponding to the tenants service host. For example, the tenants URL for the public, multitenant TACC Tapis is by https://api.tacc.utexas.edu/tenants. The corresponding top-level template directory is api.tacc.utexas.edu, with directories for each tenant therein.

To implement:

1. Add the TACC top-level directory to templates
2. Extend abaco init to accept an override tenants URL via -t
3. Add support for TAPIS_TENANTS_URL as a means of setting a global default