Allow for Pure drivers to verify HTTPS requests

Allow for Pure drivers to verify HTTPS requests

This pipes in the config options (driver_ssl_cert_verify and
driver_ssl_cert_path) for verifying https requests to the
purestorage python module. This will allow for the underlying https
management API requests to the array to be verified.

To use this feature a newer (>1.4.0) version of the pure storage python
module will be required.

DocImpact: Need to update Pure Storage Volume Driver config reference
to mention that it can use the new config options and the python module
version requirements that go along with it.

Change-Id: Id4d73e76ec64e4ac00291ed9b7377c47e40bc551
Closes-Bug: #1546655