Build wheel cache for venvs

Building a bunch of virtual envs later is going to be tedious if we do not
pre-cache certain annoying-to-build packages.

* tools/build_wheels.sh: pre-build some wheels for annoying package installs
* list distro package dependencies in files/*/venv
* list packages to pre-build as wheels in files/venv-requirements.txt
* install database Python modules when setting up the database

Change-Id: Idff1ea69a5ca12ba56098e664dbf6924fe6a2e47

lib/databases/mysql

@@ -151,6 +151,9 @@ EOF
     else
         exit_distro_not_supported "mysql installation"
     fi
+
+    # Install Python client module
+    pip_install MySQL-python
 }
 
 function database_connection_url_mysql {

lib/databases/postgresql

@@ -100,6 +100,9 @@ EOF
     else
         exit_distro_not_supported "postgresql installation"
     fi
+
+    # Install Python client module
+    pip_install MySQL-python psycopg2
 }
 
 function database_connection_url_postgresql {