Make the displayed SSH username configurable

This patch allows the SSH username displayed in templates to be
configured through the SSH_USERNAME environment variable.

capsulflask/__init__.py

@@ -58,6 +58,7 @@ app.config.from_mapping(
   LOG_LEVEL=os.environ.get("LOG_LEVEL", default="INFO"),
   SPOKE_HOST_ID=os.environ.get("SPOKE_HOST_ID", default="baikal"),
   SPOKE_HOST_TOKEN=os.environ.get("SPOKE_HOST_TOKEN", default="changeme"),
+  SSH_USERNAME=os.environ.get("SSH_USERNAME", default="cyberian"),
   HUB_TOKEN=os.environ.get("HUB_TOKEN", default="changeme"),
 
   # https://www.postgresql.org/docs/9.1/libpq-ssl.html#LIBPQ-SSL-SSLMODE-STATEMENTS