Add way to specify key-name from environ

If you're using a standard SSH keypair, it becomes tedious to have to type
`--key-name` for every novaclient command.

This patch introduces a new environment variable `NOVACLIENT_DEFAULT_KEY_NAME`
which can be used to set this value persistently.

Change-Id: I1a1180b7d474d1a4e039dbb7044cd8eaf04cc9ad
Closes-Bug: 1327418
Merge "Add way to specify key-name from environ"