Add tests for rho auth add command

[elyezer]

Added some commits to get the initial test scenarios for rho auth * commands automated:

• Created a context manager to help creating a temporary directory. This will help ensure that the data directory created by RHO gets created and removed for every test.
• Added a py.test fixture which makes use of the isolated_filesystem context manager. Now whenever a test function has the isolated_filesystem argument py.test will automatically run the context manager and generate/remove the temporary directory.
• Added pexpect as a requirement it will help running RHO and filling its inputs like the vault password.
• Finally with all the above in place, we are able to automate some test cases. These automated tests should cover the happy path and some negative tests for the rho auth * commands.

Closes #5

[coveralls]

Coverage decreased (-71.5%) to 16.498% when pulling 48afeaa on issues/5 into f7105b9 on master.

[elyezer]

ae3d149 fixes the coverage decrease, we are interested on measuring coverage only for the camayoc framework not the tests for rho.