Add `tmadmin` command

Add a `psa tmadmin <domain>` to launch the `tmadmin` command line for an app server domain. 

psa would need to: 

* set `TUXCONFIG` to `$PS_CFG_HOME/appserv/<domain>/PSTUXCFG`
* set `TUXDIR` if not set (`psadm2` has it set by default in the DPK
* verify `tmadmin` is in `PATH`

This would also help with enabling #88 