Fix using key for ssh

Before this fix manila used only password or private key from default
.ssh directory. In this patch, calling the create function will use
one of this methods for authentication. Authentication is attempted
in the following order of priority:
* The private key;
* Any key we can find through an SSH agent;
* Any "id_rsa" or "id_dsa" key discoverable in ~/.ssh/;
* Plain username/password auth.

Change-Id: I74e00dabe5b5a93fb4fbbdcfef4969971af6d0a7
Closes-Bug: #1369913
Merge "Fix using key for ssh"