Introduce a wrapper for hostname resolving

Currently a few of the LU's CheckPrereq use utils.HostInfo which raises
a resolver error in case of failure. This is an exception from the
standard that CheckPrereq should raise an OpPrereqError if the error is
in the 'pre' phase (so that it can be retried).

This patch adds a new error code (resolver_error) and a wrapper over
utils.HostInfo that just converts the ResolverError into
OpPrereqError(…, errors.ECODE_RESOLVER). It then uses this wrapper in
cmdlib, bootstrap and some scripts.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>