spell check docs with spellintian (SC-770)

[holmanb]

Fix spelling errors

- Add Makefile target that checks for spelling errors
- Add Makefile target that fixes spelling errors
- Add spelling check to travis doc tests
- Fix various spelling errors in the docs 

Test Steps

revert the bug fixes in the second commit to test the new makefile target functionality

git reset --hard HEAD^
make check_spelling
echo $?
make fix_spelling
make check_spelling # note the one error fix_spelling couldn't automagically fix
# manually fix doc/rtd/topics/datasources/smartos.rst
make fix_spelling
echo $?

Note that this build fails (before the spelling fixes) and this build succeeds (after the fixes).

[TheRealFalcon]

There goes our possibility of ever joining Hacktoberfest...

I noticed on Travis:

0.46s$ make check_spelling
Makefile:88: recipe for target '_check_spelling' failed
The command "make check_spelling" exited with 0.

I don't think the check is running correctly there. Have we tried with a misspeled word to see if Travis fails on it?

[holmanb]

#> There goes our possibility of ever joining Hacktoberfest...

We have plenty of text files with spelling errors outside of doc for the Hacktoberfesters to have a go at ;)

I noticed on Travis:

0.46s$ make check_spelling
Makefile:88: recipe for target '_check_spelling' failed
The command "make check_spelling" exited with 0.

I don't think the check is running correctly there. Have we tried with a misspeled word to see if Travis fails on it?

Fixed. This was some mostly harmless, but I came up with a slightly different solution to prevent that warning.

[TheRealFalcon]

Thanks! This will improve the quality of our docs, and hopefully make us look a little more professional (until people read my PR comments).