Add support of microversion as env var

For the moment manilaclient uses latest supported microversion if not set. And it can be set only as optional parameter in single CLI request.

It would be very useful to be able to set required microversion as we set username, project name, etc - as env variable.

Requirements for implementation:

- new env var for microversion should have lower priority than optional argument;
- manilaclient should verify that set value is supported by client itself else do not perform requests to server and raise "unsupported" error;
- name of new env var should have prefix "OS_", for example - "OS_MANILA_MICROVERSION".

Currently we have env var "OS_SHARE_API_VERSION" that is used only for getting major API version. It can be used for above described needs.