Implement get password for novaclient

This will download and decrypt a base64 encoded encrypted password
from the os-server-password extension. It depends on the user
having openssl installed, but if there is an error of any kind it
will print out the encoded and encrypted password instead.

It also implements clear_password which will delete the password
so it can no longer be retrieved.

Change-Id: I2c4e6c3f03b70dc98d6d339381648a6058f46e21

Conflicts:

	tests/v1_1/test_shell.py
ClientExceptions should include url and method

Fixes Bug 1103557.
novaclient abstracts out http request from user/client
making it unknown to user what root cause behind nova client
exceptions being raised. By including url and method in exception
handling, this allows user to handle accordingly.

Change-Id: I1a509bb932b3fd029bd0870ab699a39e21da19bb