Add 'hst:deletable' property to 'hst:mount' of channels creating during runtime

Motivation: it is logical to only delete channels created during runtime rather than ones created by bootstrapping.

Proposed solution:

• Added a boolean property 'hst:deletable' to 'hst:mount' of created channels from blueprints.
• Upon deletion, a validator will check to only allow deleting channels with hst:deletable = true.