Enhance devstack plugin

Add several features:
- possibility to set any opt to any group
- possibility to configure and enable any set of share backends

Details:
1) It is possible to set any custom opt to any config group using following:
$ export MANILA_OPTGROUP_foo_bar=value
where 'foo' is name of config group and 'bar' is name of option.

2) 'MANILA_CONFIGURE_GROUPS' contains list of config group names used to create
config groups, but 'MANILA_ENABLED_BACKENDS' is used to set config groups as
Manila share back ends. Both can be set like following:
$ export MANILA_ENABLED_BACKENDS=foo,bar
where 'foo' and 'bar' are names of config groups with opts for some share
drivers. By default they are equal. Also be attentive, if you modify both,
make sure 'MANILA_CONFIGURE_GROUPS' contains all values from
'MANILA_ENABLED_BACKENDS'.

3) Two default backends are used for compatibility with previous approach.
Two defaults backends have same configuration except name of backend.
Both use generic driver.
They can be enabled by adding values of following env vars:
'MANILA_BACKEND1_CONFIG_GROUP_NAME' and 'MANILA_BACKEND2_CONFIG_GROUP_NAME'
to the env var 'MANILA_ENABLED_BACKENDS' or will be enabled
if 'MANILA_ENABLED_BACKENDS' is empty.

Implements bp enhance-devstack-plugin

Change-Id: I910e87f00e4d309419f81469da27e70c069e6ada