Remove -p as a required parameter for SAS-backed dysks

[ehotinger]

The name of the page blob (-p, --pageblob-name) shouldn't be required for SAS-backed dysks, since it can be automatically discovered from a SAS URL. Ref: #53

[khenidak]

relevant note: The sas pushed into the module does not have path. the path is passed separately. So we will always need account/page blob name. The basic premise is we add another parameter for cloud with default to azure, Users can change its value to any of sovereign clouds