Use instance name as hostname if not provided

Based on the discussion in #66 and elsewhere, in the instance create POST we want to:

• Make hostname optional (it's currently required)
• Validate it using the same constraints we're currently using for name
• • If hostname is not provided, set name as hostname

Out of scope for now is worrying about uniqueness of hostname, e.g., if the user creates { name: 'db1', hostname: 'db2' } and then tries to create { name: 'db2' } (which implies a hostname of db2), let them.

Start at the instance create POST endpoint and locate the current hostname requirement and name validation. Make hostname optional, apply the existing name constraints, and use name when hostname is absent; uniqueness checks are explicitly out of scope. Confirm the behavior for both supplied and omitted hostname values.