Make Volume snapshot a task

Change-Id: I53fbd6e2ba9b17fa0ccd38d5e077587df0bbfef8
Clean up import of cinderclient exceptions

Don't import a particular version of the cinderclient API - just import the
client module and pass the version we want to the Client() constructor as
is intended. This allows up to access the exceptions directly through
client.exceptions, rather than having to do a separate import.

Change-Id: Ic0e47506a51a572da01fdfdef0367cfdb3e34d54
Merge "Make Volume snapshot a task"
Merge "Clean up import of cinderclient exceptions"