mgr/cephadm: enforce that a host is a valid DNS name

mgr/cephadm: enforce that a host is a valid DNS name

This combines the hostname restrictions

 * 1-63 chars
 * a-z, A-Z, 0-9, -

and the DNS name restrictions

 * .-delimited
 * no empty components (or leading or trailing .)
 * 250 chars total max

Note that this allows bare IPv4 addresses (which are indistinguishable from
a valid DNS name, AFAICS), but disallows bare IPv6 addresses.

Signed-off-by: Sage Weil <sage@redhat.com>