Add predefined variables for space domain and namespace

[FabianKramm]

Currently it is not possible to identify a space domain or namespace in the config. However it would be nice to have predefined variables for that to allow the users to customize their deployments. Domains are especially useful for ingresses. Since a space can have multiple domains, I suggest the following syntax for domains:

• DEVSPACE_SPACE_DOMAIN1, DEVSPACE_SPACE_DOMAIN2 etc.

For the namespace the variable DEVSPACE_SPACE_NAMESPACE could be used.

[FabianKramm]

Implemented in v3.5.17