Assume a Jenkins configuration with 2 SSH server configurations,
SERVER_A and SERVER_B
Assume pre-existing job A with an SSH step using SERVER_B.
Delete SSH SERVER_B from the Global Configuration, save configuration
Configure job A, change nothing and Save job.
Job config.xml now contiansthe first available value from the drop-down selector in configure.

Plugin does not warn the existing configuration is no longer available.
Plugin replaces "non-resolvable" value with first one on the list.
User is none the wiser until something goes wrong during execution.

Plugin should WARN/ERROR:
"Existing job SSH server configuration<value> is not valid"
When existing value is not found, set first value in selector list to:
" - Choose SSH server - "
Plugin should set value to - NULL - if existing value not found, not take the first on the list if job is saved.