Use an SSH pool to manage SSH connection

Use an SSH pool to manage SSH connection

Use an SSH pool to hold all SSH clients,this makes driver work when
mutiple requests received in short time.Rewrite method create() and
do not set socket timeout to be None because "None" leads to closing
ssh client blocking.

This patch alse fix:
make "Not format" to be "Notformat" in function _find_lun.
Change the type of "free_capacity_gb" fron string to be float.Because
type string will lead to cinder scheduler filter failure.

Fixes bug: 1162251
Change-Id: I7d2270bb23ccfec27e618a686ab8c278a72f2244