Removed st2web dependency on st2 module

[nmaludy]

The StackStorm.st2web role incorrectly declares a dependency on the StackStorm.st2 module. This causes issues if trying to use these roles to only install individual components on a system. Example: if you only want to install nginx and st2web on a box, it also pulls in st2.

[arm4b]

@nmaludy Could you please provide those minor changes and so we could merge the PR?

[CLAassistant]

All committers have signed the CLA.