Add support for device type in nginx helper

[pradeep910]

device_type variable was added in some sites.
For this, changes were made in nginx-helper plugin itself for the site instead of using filter / action hooks.

• Add a filter to nginx-helper plugin.

• Use filter to add enable device type.

[chandrapatel]

@pradeep910 Since actions & filters released in 2.1.0 so I'm closing this issue.