Remove Duplicate ClientExceptions and Merge

horizon handles troveclient.ClientException as a recoverable error,
with troveclient defined via "from troveclient import exceptions
as troveclient". this patch imports oslo's apiclient exceptions
in troveclient/exceptions.py to ensure all ClientExceptions are
considered recoverable. also removes unused exceptions that were
unnecessarily ported from the compat version of the client. this
fix is necessary to avoid seeing the generic error message of
"Error: There was an error submitting the form. Please try again."
in horizon vs. the explicit error message.

Change-Id: I94c8ec0289821ccd365d6689488a3e25877c1560
Closes-Bug: #1292033
Fix building of documentation

Now both the standard command "python setup.py build_sphinx" and
"tox -e docs" will build the documentation.

The changes:
* tox.ini: Fix docs section: Remove unneeded deps line, the general
  deps line is just fine. Enable the command, clean up before running
  sphinx and use the normal command for execution.
* pydocs.rst: This throw an error while building since
  troveclient.client.Dbaas does not exist, replace it.
* index.rst: Generate table of content. No need to include the files.

Change-Id: I2095ef73a76b0212e33776af925f5eaf1810be7a
Partial-Bug: #1315101
Correct grammar in trove CLI help

Fixed grammar errors such as 'a instance' and 'the instance' to be
'an instance.'

Change-Id: I43eb9562c5290d23a0def31249d6c7aefd5dc593
Closes-Bug: #1316307
Merge "Remove Duplicate ClientExceptions and Merge"
Merge "Correct grammar in trove CLI help"