ovs-vsctl-bashcomp: Avoid setting the COMP_WORDBREAKS.

ovs-vsctl-bashcomp: Avoid setting the COMP_WORDBREAKS.

Modifying $COMP_WORDBREAKS in completion script is not the recommended
as it is a global variable and the modification could affect the behavior
of other completion scripts.  As a workaround, this commit uses the
_get_comp_words_by_ref which allows user to exclude characters out of
$COMP_WORDBREAKS and reassemble input command line.  However, as a side
effect, the bash completion module cannot handle characters defined in
$COMP_WORDBREAKS (e.g. ':' and '=') correctly in the resulting completions.
Thusly, we need to trim the colon-word and equal-word prefixes from reply.

Signed-off-by: Alex Wang <alexw@nicira.com>