Clean up cached packages in st2_deploy script

This fix attempts to clean up any cached old packages. Fixes
a bug where the install may attempt to install multiple versions
of the st2 packages during an install.

Fixes: https://stackstorm.atlassian.net/browse/STORM-867
Merge pull request #837 from StackStorm/cleanup-packages-st2_deploy

Clean up cached packages in st2_deploy script