Add support for multiple environment files

This patch adds support for heat to accept multiple -e commandline
arguments. The environment files are merged, with newer values
overwriting older ones.

Change-Id: I01e2baf8dea3bafb1ac6eac34f281c5ff86bf4aa
Closes-Bug: 1350403
Removed undefined method in install_env.py file

There was a call from install_venv.py file to post_process method
of InstallVenv class, but this method is not defined in this class
as result it was raising an error. Even when this error doesn't
affect dependencies installation, it is not allowing to display user
information.

Change-Id: Idb77f164abee5625ac24800ddb94b9b8c0b85162
Closes-Bug: #1275025
Merge "Add support for multiple environment files"
Merge "Removed undefined method in install_env.py file"