Multiple ExtStorage Providers and ext-params

Add support for passing parameters to the ext template (ext-params).
Take advantage of disk-params, that don't seem to make much sense in
this template (ExtStorage Providers are not predefined and we don't
know their needs) and use them to pass the ext-params dynamically to
the template.

ext-params are correlated with gnt-os-interface's os-params.
All ext-params are exported to the ExtStorage Provider through it's
environment, with variables prefixed with 'EXTP_' (similarly to the
OS interface's 'OSP_' params).

ext-params are passed via the --disk option. If the disk template
is of type `ext', then any additional options passed to --disk and
are not in IDISK_PARAMS are considered ext-params e.g.:

 gnt-instance add -t ext --disk=0:size=2G,param1=value1,param2=value2

Finally, we introduce a new IDISK_PARAM called IDISK_PROVIDER, that is
mandatory for template `ext' and is used to select the desired
ExtStorage Provider. This parameter is not a valid --disk option for
any other template type.

The IDISK_PROVIDER parameter becomes the first element of the
disk's unique_id tuple e.g.:

 unique_id = ('sample_provider1', 'UUID.ext.diskX')

Example selecting different ExtStorage Providers for each disk and
passing different ext-params to them:

 -t ext --disk=0:size=2G,provider=sample_provider1,param1=value1
        --disk=1:size=3G,provider=sample_provider2,param2=value2

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: small simplification in bdev code, pylint fixes]
Reviewed-by: Iustin Pop <iustin@google.com>