osstest: add support for runtime_IDENT_hostflags

osstest: add support for runtime_IDENT_hostflags

This is required for FreeBSD, that will need to set some of the
hostflags at runtime. The current IDENT_hostflags will be keep as-is,
and they should only be set at job creation time.

Also introduce a helper to set the runtime hostflags.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v4:
 - Allow runtime flags to be appended instead of overwritten.

Changes since v3:
 - New in this version.