Remove hyphens from unit name when creating backend identifier.

'-' is an invalid character in VCL identifier names. This results in the following compilation error when varnish is restarted if the name of the service being related to includes a hyphen:

```
Message from VCC-compiler:
Identifier 'some-app1' contains illegal characters, use [0-9a-zA-Z_] only.
('input' Line 1 Pos 9)
backend some-app1 {
--------###########--

Running VCC-compiler failed, exit 1

VCL compilation failed
```