Add input validation to ServiceProvider

[meip]

Problem (written by @seilc1 in #219 )

We are lacking in general in the area of input validation ... I've been thinking about 3 additional functions on the ServiceProvider:

validateProvisionRequest()
validateDeprovisionRequest()
validateUpdateRequest()

this would allow us to give immediate/synchronous feedback if the request we got will be processed. This is not fool-proof as some of the checks have to be done by the backend (e.g. is there enough space). But we already can improve our service quality a lot with that.

Validation which could handled in this interface i.e.:
Validating the Parent-Child-References, I see at least the following things we should validate for Parent-Child-References:

• Does the parent exist (completed and !deleted)
• Does parent and child match together (e.g. I don't attach a MariaDB to MongoDBCluster || could also be something like can I deploy a unencrypted MariaDB to a encrypted MariaDB Cluster)
• Does the parent have enough space for another child (as done in this example ... or maybe more complex)

Requirements

• Add validation capabilities to ServiceProvider