add python-prettytable python-yaml during apt phase

To avoid error during build of the docker image
```
reparing to unpack st2client_0.6.0-7_amd64.deb ...
Unpacking python-st2client (0.6.0-1) ...
dpkg: dependency problems prevent configuration of python-st2client:
 python-st2client depends on python-prettytable; however:
  Package python-prettytable is not installed.
 python-st2client depends on python-yaml; however:
  Package python-yaml is not installed.

dpkg: error processing package python-st2client (--install):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 python-st2client
```


add missing package python-prettytable python-yaml