This commit adds the --properties-separator option, which lets users
specify the separator used between keys and values in the properties
output format. This is done by adjusting the value of
github.com/magiconair/properties#Properties.WriteSeparator at encode
time.
Some refactoring of the properties encoder unit tests was done to make
it easier to write unit tests that include different separator values.
Fixes: #1864
Signed-off-by: Ryan Drew <ryan.drew@isovalent.com>
