Key storage and management

[gschueler]

Add a facility for uploading and using SSH private keys in a secure manner.

• API for CRUD operations
• provide pluggable encryption of the stored keys
• provide pluggable backend storage of the keys
• store keys in DB or filesystem by default
• keys can be used by built-in SSH provider
• apitoken policy should allow ssh key upload by default
• gui field in project config to set default ssh key storage path
• storage config should be in rundeck-config.properties

[gschueler]

• api and nomenclature should not be ssh key specific, but instead changed to "/keys" and "key storage"