Handle read-only etc/grid/config.xml files (See #10187)

For installations which should not be modified by non-syadmins,
it is useful to allow the etc/grid/config.xml to be locked
(chmod 444). The only limitation is that the modification of
the active profile can no longer be modified via OMERO_CONFIG
or `bin/omero config def`.

All calls to `bin/omero config` which would modify config.xml
now print an error message. (`bin/omero config def` is somewhat
special since without an argument it does not modify config.xml
but with an argument it does)

Similarly, all calls to `bin/omero admin` check that either the
file is lockable or that no profile has been selected.
Handle non-default config exceptions
Merge pull request #651 from joshmoore/rebased/develop/read-only-config-xml

Handle read-only etc/grid/config.xml files (See #10187) (rebased onto develop)